fix: stabilize markdown table search and selection

[2391384896]

Summary

• Add source offset metadata to rendered table cells and mirror CodeMirror search matches into table live-preview cells through CSS Custom Highlight ranges.
• Keep table cell DOM/focus stable while typing after a cell is activated, including lower rows in long tables.
• Treat same-cell pointer drags as native text selection so selecting Float inside Floatboat, P inside P1, or Kevin inside Kevin Stratvert does not become whole-cell editing/selection.
• Defer table cell document writes until focus leaves the table, keep IME composition intermediate text out of the Markdown document, and preserve the edited row's viewport position after blur commit.

Verification

• pnpm exec vitest run packages/plugin-search/test/plugin-search.test.ts → pass (30 tests)
• pnpm exec vitest run packages/core/test/live-preview.test.ts → pass (69 tests)
• pnpm typecheck → pass
• pnpm --filter @floatboat/nexus-core build → pass
• pnpm --filter @floatboat/nexus-plugin-search build → pass
• git diff --check → pass
• AoE Electron runtime check via CDP → pass: long table row typed ABC without DOM/focus loss; Float partial selection stayed partial; Cmd+F table search selected SearchNeedle070.
• AoE Electron runtime check via CDP on /Users/zhenyu/Downloads/Floatboat table ime repro.md → pass: 第 38 行 | | aa | ... | nnasaaan | ... | 首格 composition 输入 你好 时中间态 n 不写入文档、不失焦；blur 后目标行仍停在表格可见区域，未跳到 评论样本摘录。

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