#21183 cmux-next: tune thread minimap contrast

Summary

Tune the thread minimap's resting and current tick contrast to match the Codex reference measured by PR #20560. The first pass was approximately 25/255 too bright, so quiet ticks now use 0.28 opacity and current viewport ticks use 0.48; hover, focus, and bookmarks still reach full contrast.

This is a focused follow-up to #20560 and is based on feat/thread-minimap-codex.

Validation

• git diff --check
• Static CSS regression assertions passed with Bun.
• The repository's scoped Bun test could not start because this fresh worktree has no installed @base-ui/utils dependency; no source test failure was observed.

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Summary by cubic

Tunes the thread minimap resting and current tick contrast to match the measured Codex reference. Quiet ticks now use 0.28 opacity and current viewport ticks 0.48, correcting the first pass which was about 25/255 too bright while hover, focus, and bookmark states still reach full contrast.

• Adds a static CSS regression test that locks in the reference opacities.
• Follows up on #20560 and is based on feat/thread-minimap-codex.