#21645 fix(cmux-next): restore composer row rhythm and open-chip wash (base red since #21574)

Every feat-cmux-next pull request since #21574 fails web / react-apps-check (2) on two test files it did not touch. #21574 merged with that job red (its local run skipped the browser lane). This puts back the two values that broke them, and keeps the rest of #21574.

• composer-row-rhythm.test.ts failed with uneven gaps inside groups: model→effort 19.0, steer→mic 16.0, mic→send 16.0 at all four widths. --acpmux-row-gap went from 4 px to 7 px, which only the model/effort pair uses; the trailing cluster's gap was pinned to 4 px. Back to 4 px puts model and effort 16 px apart like the trailing controls. The +/chips and mode group gaps are unchanged (the mode padding is group-gap − row-gap, so the drawn group gap stays the same).
• composer-chip-press.test.ts failed with pressed oklab(… / 0.16) against held color(srgb … / 0.11). The open-chip rule #21574 added after the ink-only hover used --acpmux-row-hover (the 11% pill hover). The rule it overrides used --acpmux-pill-open (16%), the tone the press shows. It now uses --acpmux-pill-open again, and still wins over the transparent hover.

Testing

• The failures above are from the shard 2 logs of #21574 and of #21623, #21620, #21618, #21575 and #21431. All five fail the same two files.
• Locally, the browser lane did not start: the Vite dev server stayed cold (warmDevServer: / still cold after 100000 ms) both before and after this change, so this PR's own react-apps-check (2) run is the evidence.

Changelog

none

🤖 Generated with Claude Code

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Summary by cubic

Fixes the two CSS value regressions from #21574 so web / react-apps-check (2) passes again; every feat-cmux-next PR since that merge has failed two test files it doesn't touch.

• Restores --acpmux-row-gap: 4px so model and effort sit 16 px apart, matching the trailing Interrupt, mic, and Send cluster.
• Selected/open chips now keep their --acpmux-pill-open 16% wash on hover instead of the 11% ink-only hover tone, so a press matches the held chip.