Add ASCII-art layout diagram to render.rs

Makes the nine layout constants self-documenting by showing which value
controls which part of the window (cell size, menu bar, panel margin,
side panel, and overall window dimensions).

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

src/render.rs

@@ -1,3 +1,29 @@
+// Layout constants — which value controls which part of the window:
+//
+// One board cell:
+//
+//   ◄── CELL_W (14 px) ──►
+//   ╔════════════════════╗  ▲
+//   ║         @          ║  │ CELL_H (20 px)
+//   ╚════════════════════╝  ▼
+//
+// Full window (Edit mode shown; side panel absent in Play mode):
+//
+//   ◄────────────────── DEFAULT_WINDOW_W (840 px) ───────────────────►
+//   ┌────────────────────────────────────────────────────────────────┐ ▲
+//   │ File │ Play  Edit                                              │ │ MENU_H (24 px)
+//   ├────────────────────────────────────────────────┬───────────────┤ ▼
+//   │  PANEL_MARGIN (8 px) on all sides              │  PALETTE_     │ ▲
+//   │  ┌──────────────────────────────────────────┐  │  PANEL_W      │ │
+//   │  │                                          │  │  (200 px)     │ │ DEFAULT_
+//   │  │   board cells (CELL_W × CELL_H each)     │  │  Edit mode    │ │ WINDOW_H
+//   │  │                                          │  │  only         │ │ (524 px)
+//   │  └──────────────────────────────────────────┘  │               │ │
+//   │                                                │               │ ▼
+//   └────────────────────────────────────────────────┴───────────────┘
+//
+// MIN_WINDOW_W and MIN_WINDOW_H are derived: 10 cells wide / 5 cells tall plus margins.
+
 use eframe::egui;
 use egui::{Align2, FontId, Pos2, Rect, vec2};