Fix transcript render retention for long TUI sessions

[cexll]

Summary

• Compact finalized transcript head rows after they enter native scrollback so JS render state no longer retains committed history rows indefinitely
• Keep committed compacted blocks classified as committed for removal safety checks
• Bound the Markdown render cache by approximate rendered size so very large renders are not retained as normal cache entries

Fixes #4820.

Tests

• bun test packages/coding-agent/test/modes/components/transcript-container.test.ts packages/tui/test/markdown.test.ts
• bun run check in packages/coding-agent
• bun run check in packages/tui

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[chatgpt-codex-connector]

Preserve compacted transcript rows for full repaints

When this advances #compactedChildStart, subsequent render() calls start walking children from that index, so the compacted prefix is no longer available in the rendered frame. That only works while the terminal's native scrollback still contains those rows; on a non-multiplexer resize settle the TUI explicitly requests a full repaint that clears scrollback (packages/tui/src/tui.ts:3295), and reset-display paths do the same, but the transcript can now replay only the retained tail. After a long session has compacted history, any such resize/reset permanently drops the compacted transcript rows from the terminal history instead of re-emitting the full transcript.

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[RensTillmann]

I built on this approach in #5239 and kept the original attribution. The replacement adds a NativeScrollbackReplay hook invoked before destructive full paints, allowing TranscriptContainer to rehydrate compacted children for resize/reset replay and then resume compaction on the next frame. It includes an engine-level regression covering the P1 history-loss case, plus the transcript and Markdown cache tests.