fix(search): Preserve Chinese IME composition

[Xuepoo]

Summary

• yield document-level search navigation keys while native IME composition is active
• retain keyCode 229 as a compatibility fallback for process-key events
• route the production document handler through a testable state/action contract
• preserve ArrowUp, ArrowDown, Enter, and Escape behavior after composition ends

Closes #48

Validation

• CI bun test — 29/29, 440 assertions; includes 15 production search-routing tests
• bun run check
• bun run lint:md — 578 Markdown files, 0 issues
• bun run build — 562 pages / 22 sections
• Playwright projected-input flow: Chinese value 中文 survives composition; candidate keys are not prevented; normal Escape closes search after compositionend
• independent final review: no findings; PX-0153 resolved by the ordered rebase and CI run

Scope

The task diff contains only src/nav.ts and src/nav.test.ts. It is based on main commit 103107c and inherits the Bun test gate introduced by PR #49.