docs: scope Microban credit to level layouts only

Reword README, PDR, menu scene, and the data file header so the Microban
credit clearly covers only the puzzle layouts borrowed from David W.
Skinner. The parser, renderer, UI, and progression code are original, and
ownership is now stated explicitly. Attribution for Skinner is preserved
everywhere it appeared.

src/game/data/microban-levels.js

@@ -1,7 +1,9 @@
 /**
- * Microban level set by David W. Skinner (April 2000, 155 puzzles).
- * Freely distributable with credit. First 100 levels shipped here — small,
- * concept-focused puzzles that are great for beginners and still interesting.
+ * Puzzle layouts from the Microban level set by David W. Skinner
+ * (April 2000, 155 puzzles, freely distributable with credit).
+ * First 100 layouts shipped here — small, concept-focused puzzles.
+ * Only the level designs are borrowed; the parser, renderer, and engine
+ * are original code written for this project.
  * Source: http://users.bentonrea.com/~sasquatch/sokoban/
  *
  * Format: XSB

src/game/scenes/menu-scene.js

@@ -19,7 +19,7 @@ export default class MenuScene extends Phaser.Scene {
         const cy = this.cameras.main.centerY;
 
         this.add.text(cx, cy - 200, 'SOKOBAN', FONTS.title).setOrigin(0.5);
-        this.add.text(cx, cy - 130, '100 Microban puzzles', FONTS.subtitle).setOrigin(0.5);
+        this.add.text(cx, cy - 130, '100 puzzles to solve', FONTS.subtitle).setOrigin(0.5);
 
         createButton(this, cx, cy - 20, 'PLAY', () => this.scene.start('LevelScene'), { width: 240, height: 64 });
 
@@ -33,6 +33,6 @@ export default class MenuScene extends Phaser.Scene {
         ].join('\n');
         this.add.text(cx, cy + 170, hints, { ...FONTS.small, align: 'center' }).setOrigin(0.5);
 
-        this.add.text(cx, this.cameras.main.height - 24, 'Microban puzzles by David W. Skinner', FONTS.small).setOrigin(0.5);
+        this.add.text(cx, this.cameras.main.height - 24, 'Level layouts from Microban by David W. Skinner', FONTS.small).setOrigin(0.5);
     }
 }