fix: stabilize markdown table search and selection

[2391384896]

Summary

• Mirror CodeMirror search matches into rendered table cells with CSS Custom Highlight ranges backed by exact Markdown-source mappings.
• Keep table cell DOM/focus stable while typing, including long-table IME composition and partial native text selection.
• Preserve pending cell edits across immediate blur, row/column reorder, setDocument(), and editor teardown without stale-range writes or data loss.
• Isolate table editing locks and search highlight registries per editor instance.
• Handle tables without leading pipes, escaped \| delimiters, rich inline Markdown, repeated link labels/URLs, multi-backtick inline code, and full-document regexp context.

Behavior note

• setDocument() and destroy() now flush a pending table-cell edit before replacement/teardown. A non-silent pending edit is emitted through onChange before the lifecycle operation completes.

Verification

• pnpm test → pass (537 tests)
• pnpm typecheck → pass
• pnpm build → pass
• pnpm build:electron-demo → pass
• pnpm exec vitest run packages/core/test/editor.test.ts packages/core/test/live-preview.test.ts packages/plugin-search/test/plugin-search.test.ts → pass (159 tests)
• git diff --check → pass
• Previous AoE Electron runtime checks remain covered: long-table typing retained DOM/focus; partial selection stayed partial; Cmd+F selected the rendered table match; IME intermediate text stayed out of Markdown until commit.

Compliance

• AI assistance disclosure: Codex assisted with reproducing edge cases and drafting the follow-up fixes and regression tests. Human maintainer review is requested and required before merge.
• No new dependencies, build artifacts, secrets, or personal document data are included.

[CLAassistant]

All committers have signed the CLA.