## Summary

Add browser-font fallback to GPUI on the web, so applications can render
emoji using the user’s local fonts without bundling an emoji font.

GPUI continues to shape text with Cosmic Text using application-provided
fonts. When those fonts cannot supply an eligible grapheme or its
requested emoji presentation, the web backend measures and rasterizes
the complete grapheme using Canvas 2D, then caches and paints the result
through GPUI’s existing glyph atlas.

### Configurable fallback

- **Emoji-only by default**, including supported variation-selector,
skin-tone, flag, keycap, and ZWJ sequences.
- Applications can opt into **CJK fallback** for a conservative subset
of horizontal Han, kana, and modern Hangul text.
- Applications can disable Canvas fallback entirely.

CJK fallback renders graphemes independently. It prioritizes readable
text over full fidelity to contextual spacing and font features. Browser
font coverage and appearance depend on the user’s environment.

### Web input and application lifecycle

- Fix text insertion that arrives without a corresponding keyboard
event, allowing emoji input through the browser’s text-input path.
- Keep the GPUI application alive after `Application::run` returns on
WebAssembly. Browser startup is asynchronous; releasing the application
after the startup callback previously caused the window to disappear.

### Browser example gallery

Make the existing GPUI examples available from a single development
server, with separate routes for hello world, styled text, text layout,
wrapping, input, and the prime sieve demo.

The gallery supports direct links and reloads, loads only the selected
example, and provides a simple way to exercise font fallback and text
input.

## Validation

- Native unit tests cover fallback eligibility and presentation, policy
defaults, glyph-cache reuse, and in-place glyph-run replacement.
- All six gallery binaries pass WASM compilation checks.
- Browser smoke coverage exercises emoji insertion, whole-grapheme
Canvas rendering, cache reuse, and deletion.
- Manually exercised the gallery and fallback rendering in Chrome.

Release Notes:

- N/A