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>

maps/start.toml

@@ -5,9 +5,9 @@ height = 25
 player_start = [30, 12]
 
 [palette]
-" " = { archetype = "empty", ch = " ", fg = "#000000", bg = "#000000" }
-"#" = { archetype = "wall",  ch = "#", fg = "#808080", bg = "#606060" }
-"+" = { archetype = "banana",  ch = "#", fg = "#808080", bg = "#606060" }
+" " = { archetype = "empty", tile = " ", fg = "#000000", bg = "#000000" }
+"#" = { archetype = "wall",  tile = "#", fg = "#808080", bg = "#606060" }
+"+" = { archetype = "banana", tile = "#", fg = "#808080", bg = "#606060" }
 
 [grid]
 content = """