feat: show physics on load and quiet the chrome

[chowjiaming]

Summary

• Add fill_rect / paint_line plus demo scenes (Sink, Float, Burn, Wind, Glass); cold load starts on Burn so the physics is visible immediately.
• Move probe/FPS onto the canvas, drop kbd chips, and add a ? shortcut overlay, loading/empty states, and a11y nits (aria-pressed, :focus-visible, reduced-motion).
• Shift-drag snaps strokes to H/V/45°, the brush ghost tints to the active swatch, and the canvas uses integer scale (fractional only when the window is smaller than 1×).

Test plan

• just verify (fmt, clippy -D warnings, tests, wasm-pack --target web)
• First load shows the Burn scene (wood + fire), not an empty grid
• Sink / Float / Burn / Wind / Glass each clear and stamp the expected setup
• Clear shows “Paint, or pick a scene” until the next stroke or scene
• Shift-drag paints a straight H/V/45° line; ghost matches the snapped segment and swatch color
• Probe (bottom-left) and fps/tps (bottom-right) overlay the playfield; ? toggles shortcuts and auto-hides after 8s or first paint
• index.html still imports ./pkg/sandboxed.js (relative)

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