Add browser debugging support

docs/debugging.md

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+# Debugging
+
+## VS Code
+
+1. Open this folder in VS Code.
+2. Run `Debug Heros Web (Chrome)` from Run and Debug.
+3. VS Code starts the Vite dev server and opens Chrome at `http://localhost:5173`.
+4. Set breakpoints in `src/game/scenes/*.ts`.
+
+## Browser Console
+
+Development builds expose:
+
+```js
+window.__HEROS_GAME__
+window.__HEROS_DEBUG__
+```
+
+Useful commands:
+
+```js
+__HEROS_DEBUG__.activeScenes()
+__HEROS_DEBUG__.battle()
+__HEROS_DEBUG__.goToBattle()
+__HEROS_DEBUG__.toggleBattleOverlay()
+```
+
+## Battle Hotkeys
+
+- `F9`: Toggle battle debug overlay.
+- `F10`: Log battle state to the browser console.
+
+The battle debug state includes turn, phase, selected unit, pending move, acted units, unit coordinates, attack intents, and bond progress.