Add bitmap font rendering via PNG tilesheet

Replace egui text rendering with a BitmapFont system:
- Glyph.tile: u32 replaces ch: char; top-left pixel of PNG is background sentinel
- BitmapFont loads/preprocesses PNG to opaque-white + transparent, tinted at render time
- Default font: assets/vga-font-8x16.png (CP437, 8×16 tiles); placeholder generated if missing
- Boards can specify a per-board font via [font] in their .toml file
- Editor Board tab: "Font…" button opens font dialog (rfd file picker, tile dims, tilesheet preview)
- Glyph picker: tile grid from active font replaces hardcoded ASCII char grid
- map_file: TileIndex untagged enum accepts char or integer in palette entries

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -45,46 +45,68 @@ The game is a single Rust binary using **eframe 0.33 / egui 0.33** for the GUI.
 ### Modules
 
 **`src/game.rs`** — all core game types:
-- `Glyph` (`Copy`) — per-cell visual: `ch: char`, `fg/bg: Color32`. `Glyph::player()` is the only constructor used at runtime (colors for board tiles come from the map file).
+- `Glyph` (`Copy`, `Eq`, `Hash`) — per-cell visual: `tile: u32` (tilesheet index), `fg/bg: Color32`. `Glyph::player()` is a `const fn`; all other glyphs come from the map file. Derives `Hash` so boards can deduplicate glyphs into a palette.
+- `FontSpec` — optional per-board bitmap font override: `path: String`, `tile_w: u32`, `tile_h: u32`. When `None`, the app default font is used.
 - `Behavior` — plain data struct of runtime behavioral properties: `passable: bool`, `opaque: bool`. Returned by `Archetype::behavior()`; new properties added here require no match arms elsewhere.
 - `Archetype` (`Copy`, `PartialEq`) — enum of named element types: `Empty`, `Wall`, `Object`, `ErrorBlock`. Each variant provides `behavior()`, `name()` (used in map files), and `default_glyph()` (used by the editor when stamping a cell). `ErrorBlock` is a sentinel for unknown archetype names — renders as yellow `?` on red.
 - `ALL_ARCHETYPES: &[Archetype]` — ordered list of valid editor choices (excludes `ErrorBlock`).
-- `Board` — the complete game unit (ZZT-style "board"): `width`, `height`, `cells: Vec<(Glyph, Archetype)>` (row-major; each cell owns its visual and behavioral class directly), `player: Player`, `objects: Vec<ObjectDef>`, `portals: Vec<PortalDef>`. `cells` is `pub(crate)`.
+- `Board` — the complete game unit (ZZT-style "board"): `width`, `height`, `cells: Vec<(Glyph, Archetype)>` (row-major; each cell owns its visual and behavioral class directly), `player: Player`, `objects: Vec<ObjectDef>`, `portals: Vec<PortalDef>`, `font: Option<FontSpec>`. `cells` is `pub(crate)`.
 - `Player` — `x: i32, y: i32`
 - `ObjectDef` / `PortalDef` — parsed from map files, stored on Board; not yet runtime-wired
 - `GameState` — holds `board: Board`; `try_move(dx, dy)` checks passability before moving the player
 
+**`src/font.rs`** — bitmap font loading and UV mapping:
+- `BitmapFont` — wraps an egui `TextureHandle` (preprocessed to opaque-white / transparent) plus tile dimensions. The top-left pixel of the source PNG defines the "background" color; those pixels become `TRANSPARENT`, all others become `WHITE`. At render time, `painter.image(…, tile_uv, fg_color)` tints white pixels to `fg` and transparent pixels reveal the `bg` fill behind.
+- `BitmapFont::load(ctx, path, tile_w, tile_h)` — loads from disk
+- `BitmapFont::from_bytes(ctx, bytes, tile_w, tile_h, label)` — loads from embedded bytes
+- `BitmapFont::create_placeholder(ctx)` — 16×16 grid of 8×8 procedural tiles (bit-pattern bars), used as fallback when no font file is present
+- `BitmapFont::tile_uv(tile)` — returns a UV `Rect` for the given tile index (left-to-right, top-to-bottom). Unit-tested via `compute_tile_uv`.
+- `BitmapFont::tile_cols()`, `tile_count()`, `img_w()`, `img_h()` — geometry helpers used by the glyph picker and font dialog
+
+**`src/font_dialog.rs`** — font picker dialog:
+- `FontDialogState` — in-progress edit: `path: String`, `tile_w/tile_h: u32`, `preview: Option<BitmapFont>`
+- `FontDialogState::from_spec(spec)` — initializes from an existing `FontSpec` or defaults (8×16, empty path)
+- `show(ctx, open, state, font_spec)` — floating dialog with file browser (`rfd::FileDialog`), tile dimension `DragValue` controls, scrollable tilesheet preview with grid overlay, and Apply / Use default buttons. Uses a `should_close` flag to avoid the egui `.open()` borrow conflict.
+
 **`src/map_file.rs`** — map file loading:
 - `MapFile` and friends — serde `Deserialize` types for TOML map files
-- `impl From<MapFile> for Board` — converts a parsed file into a ready-to-use `Board`
+- `TileIndex` — `#[serde(untagged)]` enum accepting either `Num(u32)` or `Chr(char)`; lets map files use `tile = " "` (char) or `tile = 32` (integer) interchangeably
+- `FontHeader` — deserializes the optional `[font]` section (`path`, `tile_w`, `tile_h`)
+- `impl From<MapFile> for Board` — converts a parsed file into a ready-to-use `Board`; unknown archetype names produce `ErrorBlock`
 - `pub fn load(path: &str) -> Result<Board, Box<dyn std::error::Error>>` — reads and converts a `.toml` map file
 
 **`src/main.rs`** — app entry point and frame loop:
 - `AppMode` enum (`Play` | `Edit`) — gates arrow-key input; toggles the side panel and viewport mode
-- `App` holds `GameState`, `AppMode`, and `EditorState`; `update` phases: input → menu bar → editor panel → board → glyph picker dialog
+- `App` holds `GameState`, `AppMode`, `EditorState`, `default_font: BitmapFont`, and `board_font: Option<BitmapFont>`
+- `App::new(board, egui_ctx)` — loads `assets/vga-font-8x16.png` as the default font (falls back to `create_placeholder`); loads per-board font from `board.font` if present
+- Font change detection: `font_spec_before` is snapshotted before `show_editor_panel`; if changed, `apply_font_spec` reloads `board_font`
+- Borrow split: `let active_font = self.board_font.as_ref().unwrap_or(&self.default_font)` is extracted before mutable borrows of `editor` and `board.font` to satisfy the borrow checker
 - **Play mode**: arrow keys move player; `render::board_origin` centers or player-tracks the viewport
 - **Edit mode**: `ScrollArea::both()` wraps the board; click-to-paint stamps `(editor.glyph, editor.selected)`; calls `editor::show_editor_panel` and `glyph_picker::show`
 
-**`src/render.rs`** — cell rendering constants and drawing primitives:
-- `CELL_W = 14.0`, `CELL_H = 20.0` and window sizing constants (`DEFAULT_WINDOW_W = 840`, `DEFAULT_WINDOW_H = 524`)
-- `draw_glyph(painter, origin, x, y, glyph)` — filled rect (bg) + centered monospace char (fg)
-- `draw_board(painter, origin, board)` — draws all cells then player overlay
-- `board_origin(available, board_w, board_h, player)` — centers board or clamps to player with no empty space
-- `pos_to_cell(origin, pos) -> (i32, i32)` — pixel → cell coordinates via floor division; negatives signal out-of-bounds
+**`src/render.rs`** — cell rendering and drawing primitives:
+- Window sizing constants (`DEFAULT_WINDOW_W = 840`, `DEFAULT_WINDOW_H = 524`, `MIN_WINDOW_W/H`); no fixed `CELL_W/H` — cell size comes from the active `BitmapFont`
+- `paint_glyph(painter, rect, glyph, font)` — `rect_filled` with `glyph.bg`, then `painter.image` tinted by `glyph.fg`
+- `draw_glyph(painter, origin, x, y, glyph, font)` — sizes cell from `font.tile_w/tile_h`, delegates to `paint_glyph`
+- `draw_board(painter, origin, board, font)` — draws all cells then player overlay
+- `board_origin(available, board_w, board_h, player, font)` — centers board or clamps to player with no empty space
+- `pos_to_cell(origin, pos, tile_w, tile_h) -> (i32, i32)` — pixel → cell coordinates; negatives signal out-of-bounds
 
 **`src/editor.rs`** — editor state and side panel:
 - `EditorTab` enum (`Palette` | `Board` | `World`) — which tab is active
-- `EditorState` — holds `selected: Archetype`, `glyph: Glyph`, `glyph_picker_open: bool`, `tab: EditorTab`; selecting a new archetype resets `glyph` to that archetype's default
-- `show_editor_panel(ctx, editor, board)` — resizable right-side panel (default 200 px); Palette tab shows archetype list and glyph preview button; Board/World tabs are placeholders
+- `EditorState` — holds `selected: Archetype`, `glyph: Glyph`, `glyph_picker_open: bool`, `tab: EditorTab`, `font_dialog_open: bool`, `font_dialog_state: FontDialogState`; selecting a new archetype resets `glyph` to that archetype's default
+- `show_editor_panel(ctx, editor, font_spec, active_font)` — resizable right-side panel (default 200 px); Palette tab shows archetype list and glyph preview button; Board tab shows current font path and "Font…" button that opens `font_dialog::show`; World tab is a placeholder
 
 **`src/glyph_picker.rs`** — floating glyph picker dialog:
-- `show(ctx, open, glyph, board_cells)` — takes `open: &mut bool` and `glyph: &mut Glyph` directly; no dependency on `EditorState`
-- Three sections: board palette (unique glyphs, click to select), FG/BG color pickers, 16×6 printable ASCII character grid
+- `show(ctx, open, glyph, board_cells, font)` — takes `open: &mut bool` and `glyph: &mut Glyph` directly; no dependency on `EditorState`
+- Three sections: board palette (unique glyphs deduped via `HashSet<Glyph>`, click to select), FG/BG color pickers, scrollable tile grid (all tiles in the active font at `tile_w × tile_h` per cell)
 
 ### Map file format (`maps/*.toml`)
 
 XPM-inspired: a `[palette]` maps single characters to `(Glyph, Archetype)` definitions; `[grid] content` is a TOML multi-line string where each character indexes the palette.
 
+**Keep this example in sync with `map_file.rs` whenever the format changes.**
+
 ```toml
 [map]
 name = "Room Name"
@@ -92,9 +114,15 @@ width = 60
 height = 25
 player_start = [30, 12]
 
+# Optional: override the default font for this board.
+[font]
+path = "assets/my_font.png"
+tile_w = 8
+tile_h = 16
+
 [palette]
-" " = { archetype = "empty", ch = " ", fg = "#000000", bg = "#000000" }
-"#" = { archetype = "wall",  ch = "#", fg = "#808080", bg = "#606060" }
+" " = { archetype = "empty", tile = " ", fg = "#000000", bg = "#000000" }
+"#" = { archetype = "wall",  tile = 35,  fg = "#808080", bg = "#606060" }
 
 [grid]
 content = """
@@ -117,7 +145,7 @@ target_map = "cave"
 target_entry = "west_door"
 ```
 
-Colors are `"#RRGGBB"` hex strings. `player_start` is a header field — the player is not a board cell. The grid multi-line string's leading newline is trimmed by TOML; trailing newline is handled correctly by `str::lines()`. Unknown archetype names produce an `ErrorBlock` cell and a logged warning.
+Colors are `"#RRGGBB"` hex strings. `player_start` is a header field — the player is not a board cell. The `tile` field accepts either a single-character string (`tile = " "`) or an integer (`tile = 35`); both are valid and existing char-style map files continue to work. The grid multi-line string's leading newline is trimmed by TOML; trailing newline is handled correctly by `str::lines()`. Unknown archetype names produce an `ErrorBlock` cell and a logged warning.
 
 ### Key design decisions