Render Codex conversations incrementally

[VolkerChristian]

Summary

• apply verified agent-message and command-output appends directly to existing widgets
• keep agent output plain for the complete active/non-terminal turn, even when an item reports a terminal-looking status
• preserve the same streaming widget across unrelated full refreshes and perform one sanitized Markdown render at the terminal turn boundary
• measure speculative height-for-width queries in a non-visual document so layout probes never reflow or repaint the visible stream
• expose height-changing stream batches atomically after final geometry and tail pinning
• reuse timeline segments, relayout only for geometry changes, and freeze presentation while the reader is off the bottom
• keep collapsed activity reasoning and output lazy until disclosure

Validation

• renderer CodexUI suite passes 6/6 locally
• exact-current isolated 16 ms live stream: 271 appends, exactly one visible paint per append, stable widget identity, and one terminal Markdown promotion
• live scroll trace: no speculative-width cycles, no scrollbar regression, and no off-bottom sample while following the tail
• regressions cover visible-document measurement isolation, cursor reconciliation, geometry-change-only invalidation, atomic final-frame exposure, and terminal Markdown promotion

Stack

• base PR: Move Codex frontend processing off the UI thread #35