#287 Tab strip: stop per-render context-menu, hint-monitor and animation work

Summary

Sentry reports hangs of 8 s or more with the main thread rendering the pane tab strip: CMUXTERM-MACOS-41NN (new in cmux 0.65.1) and about 800 earlier events with the same TabItemView.body → tabControlShortcutHintVisibilityAnimation frame. Related groups sample the same render path: -2QV2 and -2QAN (context-menu state per tab), and -2SCZ, -2PRX and -26QQ (hint monitor initialization reading user defaults).

This change removes avoidable work from the tab strip and adds a low-volume diagnostic seam for the remaining render multiplier:

• Tab context-menu state is built when a menu opens, instead of for every tab on every render. That stops calling host providers once per tab per render.
• The shortcut-hint monitor no longer reads and decodes the shortcut setting from user defaults during every tab-bar rebuild.
• The whole-tab hint .animation is removed. The hint fade keeps its own animation.
• A host callback emits one sampled TabBarRenderDiagnostic render window when tab-bar body evaluations stay continuously active for at least 500 ms and 32 evaluations. cmux records that as a deferred Sentry breadcrumb with render, tab and pane counts.

Hint visibility and appearance are unchanged.

Testing

• TabBarRenderWorkTests.testRenderingTabsDoesNotQueryContextMenuProviders was committed before the fix and failed with 78 host-provider calls; it passes with the lazy state change.
• TabBarRenderWorkTests.testMenuBuildEvaluatesTabStateOnce verifies menu state is evaluated only when the menu is built.
• TabBarRenderWorkTests.testSustainedTabBarRendersEmitOneDiagnostic verifies the continuous-render threshold and one-shot sampling behavior.
• CI: all Bonsplit tests pass on the hosted macOS runner.

Changelog

Fixed: Pane tab bars do less work on every redraw and report sustained render storms for hang diagnosis

Proof

CI only; no visual change.