perf(tui): avoid reparsing frozen streaming markdown

[TheGreatAxios]

Summary

• Preserve settled markdown prefixes while append-only assistant text streams
• Repaint frozen markdown after edits, boundary moves, resizes, and stream finalization
• Clear per-row incremental state when the stream closes

Verification

• bun run check passes
• The pre-fix N/2N benchmark records 20/40 frozen assignments, 6,380/25,560 assigned characters, and a 4.006x ratio; this branch records 0/0 assignments, 0/0 characters, and a 1x ratio

Fixes CL-9011

[linear-code]

CL-9011

[TheGreatAxios]

Closing after the 0333 merge wave: rebase onto origin/main hit conflicts, and parent guidance is not to fight those. Reopen on a fresh branch if this still improves operator output without regression.