examples(surface): comprehensive Surface demo on a single screen

[re-marked]

A rewrite of `examples/surface/surface.tsx` (now `pnpm demo:surface`) that puts every Surface aspect on one labeled screen, with interactive controls that exercise the edge cases and the codex-driven fixes from #90:

#	Section	What it shows
1	LAYERS	All 8 named bands (`base / docked / overlay / dropdown / modal / popover / tooltip / drag-ghost`) painted in a diagonal stack — paint order matches band ascending. Each surface is labeled with its layer name + numeric z.
2	ELEVATION (0…5)	Six surfaces side-by-side at each elevation level. Shadow band grows wider as the level rises.
3	BOUNDARY	Two overlapping clickable Surfaces with click counters. The boundary surface (top, `hitTestBoundary`) occludes the sibling everywhere it covers, but click-through works on the sibling's exposed left + right edges.
4	COMPOSITION	`` inside an elevated `` — drag-time z boost still wins paint over the parent's chrome and elevation shadow.
5	BOX/SURFACE PARITY	Bare `` next to a bare `` with identical props — visually indistinguishable (regression guard for the codex P1 Box-defaults fix).
6	STRESS	Movable / resizable / elevation-adjustable Surface (arrows / -+= / [] keys). Exercises the shadow-clear-on-move/shrink path (codex P2 fix on #90).
7	MODAL+BACKDROP	Press `m` to toggle a modal with auto-rendered backdrop. `n` toggles a nested modal at `z=3010` — each modal pairs with its own scrim at `(z - 1)`, demonstrating peer-modal layering.

Keys

```
q · Esc · Ctrl+C quit
m toggle modal (with backdrop)
n toggle a nested modal (peer above the first)
←→↑↓ move the stress-test surface

• / = shrink / grow the stress-test surface
  [ / ] decrease / increase its elevation
  ```

Notes

• One screen, no tabs / scrolling — every aspect visible at once.
• Interactive controls intentionally exercise the codex-fixed paths (shadow clean-up on move/shrink, Box flex defaults via parity demo).
• Boundary section has live click counters so you can verify occlusion + click-through with a few real clicks rather than trusting the test pin.

Test plan

• `pnpm typecheck` / `pnpm lint` / `pnpm test` all clean (863 tests pass, unchanged)
• `pnpm build` clean
• Demo boots in a real TTY (verified through to AlternateScreen mount; piped stdin can't enter raw mode, expected)

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