fix(renderer): keep half the terminal when the flex reserve would take more - #702

Relates to #498.

What

The width a flex mode reserves to the right of the status line now never takes more than half the terminal. Full width minus 40, and Full width until compact past its threshold, still reserve 40 columns from 80 columns up, so wide terminals are unchanged. On narrower terminals the status line keeps half the width:

Full width always reserves 6 columns, so it only changes below 12 columns.

Why

#498 asks for a status line that works on a phone (the Moshi iOS app), where the terminal is about 40 columns wide. With the 40-column reserve that left almost nothing:

• At 41 columns the whole line truncated to a single ., and at 60 to one widget.
• At 40 columns or fewer the line has rendered empty since the zero-width fix (fix(renderer): truncate at zero width, keep FLEX text, no custom colors at No Color #678). Before that fix it printed at full length and wrapped, which is what the issue's screenshot shows.

The reserve is there for Claude Code's auto-compact message, which can't fit beside anything on a terminal that narrow anyway, so giving it all of the width only costs the status line.

How

resolveEffectiveTerminalWidth in renderer.ts reserves min(reserve, floor(width / 2)) columns instead of the full reserve, for every flex mode, in the piped render and the TUI preview alike, so the preview still matches what Claude Code receives. The line can no longer get a width of 0 from a detected width, so a stale comment about that went too. docs/USAGE.md says how Full width minus 40 behaves below 80 columns.

This doesn't make layouts responsive, which the issue also asks about (for example dropping widgets on narrow terminals). It fixes the empty line and keeps half the width.

Demo

The same line piped with Full width minus 40 at 120 down to 30 columns, each render wrapped in [ ] so an empty one shows.

Powerline: before

Powerline: after

Plain: before

Plain: after

Testing

• renderer-flex-width.test.ts:
  • New: every flex mode at 6 to 120 columns, both sides of Full until compact's threshold, piped and preview (20 cases), and Powerline with a flex separator at 30 to 120 columns.
  • These replace the two tests that pinned the empty line at 40 columns or fewer.
  • The tests that check the 40-column reserve itself now run at 100 columns with a longer line, since 50 columns is now in the capped range.
• bun test: 3112 pass, 0 fail. bun run lint passes.
• The changed test file under Node (Vitest): 28 of 28 pass.
• Built CLI under Bun 1.4.2 and Node 26.10.0, Full width minus 40 at 120, 80, 60, 41, 40 and 30 columns, plain and Powerline: both runtimes give the same output, identical to main at 120 and 80 columns, and matching the table above at the narrower widths. runtime-check.sh against main with the baseline configs: same output, and the TUI opens and exits cleanly under both.