fix: render strikethrough text inside markdown table cells

[2391384896]

Summary

• Treat Lezer StrikethroughMark nodes as inline markdown markers so GFM delete text inside table cells renders without raw ~~ delimiters.
• Extend the live-preview table inline formatting test to cover delete/strikethrough cells.

Verification

• pnpm exec vitest run packages/core/test/live-preview.test.ts -> pass (63 tests)
• pnpm --filter @floatboat/nexus-core build -> pass
• git diff --check -> pass

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