fix(coding-agent): bound compaction status width

[minpeter]

Summary

• keep the merged fix(coding-agent): stabilize compaction lifecycle #310 CompactionStatusIndicator architecture
• reserve terminal width for both status and useful streamed preview
• truncate the combined ANSI/wide-glyph-aware row to the exact render width
• prevent progress text from wrapping into rows that move the composer and viewport

This one-commit PR is based directly on merged #310/current main. It supersedes #322, whose pre-merge implementation
used a separate progress component that is obsolete after #310.

RED -> GREEN

• RED on exact merged main:
  • a hostile multiline progress update rendered at visible width 606 for a requested 40-column terminal row
• GREEN:
  • exactly one rendered line
  • visible width <= 40
  • compaction status and useful normalized preview both remain visible
  • complete interactive compaction file: 10/10

Verification

• root build: passed
• root npm run check: passed
• real 80x24 source TUI with local fake provider:
  • three live turns
  • manual /compact
  • one-row active compaction indicator
  • transcript and bottom status retained
  • exit 0
• browser xterm.js evidence:
  • xterm-live/terminal.png
  • xterm-live/terminal.txt
  • xterm-live/metadata.json
• fake provider, PTY, Chrome, TCP port, HOME/config/session sandbox all cleaned

---

Summary by cubic

Bounds the compaction status row to the terminal width so progress text never wraps or shifts the composer/viewport. Spinner, status, and streamed preview now render on a single line with ANSI/wide-glyph aware truncation.

• Bug Fixes
  • Truncate combined status + preview to the exact width using visibleWidth/truncateToWidth from @earendil-works/pi-tui.
  • Allocate width between status and progress to keep both visible; clamp to one rendered row.
  • Update test to assert a hostile multiline update renders as one line within 40 cols; refresh changes.md.

^{Written for commit 49176d5. Summary will update on new commits.}