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.

docs/project-overview-pdr.md

@@ -1,7 +1,7 @@
 # Sokoban — Product Development Record
 
 ## What it is
-Browser-based Sokoban puzzle game. 100 solvable Microban levels (David W. Skinner, freely distributable), Phaser 3 + Vite, deployed to GitHub Pages.
+A from-scratch Sokoban implementation by tiennm99. Phaser 3 + Vite, deployed to GitHub Pages. The engine, UI, parser, and progression system are original code. Puzzle layouts come from David W. Skinner's freely distributable **Microban** set (100 of his puzzles shipped) — layouts only, no code reused from Skinner's own programs.
 
 ## Goals
 - Clean, beginner-friendly Sokoban that runs in any modern browser with no install.
@@ -24,5 +24,6 @@ Casual puzzle fans, Sokoban beginners. Microban set chosen specifically because
 - Runs smoothly on desktop browsers at 1024×768 canvas (scaled responsively).
 
 ## Credits
-- Puzzles: Microban by David W. Skinner (April 2000). Used per the collection's free-distribution terms with credit.
+- Puzzle layouts: Microban by David W. Skinner (April 2000). Used per the collection's free-distribution terms with credit. Only the level designs — none of Skinner's code.
+- Game code: tiennm99 (with AI pair-programming from Claude).
 - Engine: Phaser 3.