feat: ship 100 Microban levels with overhauled UI and modular core

Replace the 3 hand-crafted JSON levels with the full Microban set (100 solvable
puzzles by David W. Skinner) stored as XSB text and parsed at runtime. Rework
the game into kebab-case modules under a 200-LOC budget:

- core/level-parser: XSB parse with flood-fill interior detection
- core/board-model: pure move/undo/win logic, Phaser-free
- core/progress-store: localStorage persistence with graceful fallback
- core/theme: Nord palette, fonts, responsive tile sizer
- ui/button-factory: one rounded button impl with hover/press
- ui/board-renderer: animated tile/wall/goal/box/player drawing
- scenes: menu, paginated level select (5x4 grid, 5 pages), game

Add WASD movement, U/Z undo, R restart, Esc to menu, live move counter,
best-move tracking per level, win overlay with next/levels actions, and a
radial-gradient CSS backdrop.

Drop the dead Arcade Physics wiring, the broken manual shutdown/destroy code,
the unused main.js self-import, and the hardcoded 3-level registry state.

Add docs/ and refresh README.

index.html

@@ -5,7 +5,7 @@
     <link rel="icon" type="image/png" href="./favicon.png" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <link rel="stylesheet" href="./style.css">
-    <title>Sokoban - A Phaser Game</title>
+    <title>Sokoban — 100 Microban Puzzles</title>
 </head>
 
 <body>