Summary: Improves Composer commit selection responsiveness when working with very large diffs by decoupling selection highlight rendering from expensive diff/file processing.

Changes:

• Memoizes `updateHunkAssignments` in ComposerApp to avoid recomputing assignments when hunks/commits references are unchanged.
• Adds internal selection state to gl-commits-panel so the highlight updates immediately on click, then dispatches selection events after paint.
• Introduces a double-requestAnimationFrame helper to delay event dispatch until the selection frame has been rendered.
• Caches per-commit additions/deletions and file counts, recomputing only when commits or hunks change.
• Defers gl-diff-file diff parsing via scheduled macrotasks and reprocesses when the element becomes visible to reduce main-thread blocking.
• Removes the selection transition to eliminate perceived highlight lag.

Technical Notes: The delayed dispatch pattern ensures UI feedback is painted before the details panel begins heavy diff rendering, and scheduled diff parsing reduces work in Lit’s microtask update batch.

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