perf(core): stop rescanning stable content on scroll-only frames

[GeekCmore]

Summary

• Root cause: pi-tui rebuilds its render cache every frame, so a wheel scroll re-rendered the whole content tree — and FrameClampedContainer re-ran visibleWidth over every transcript row (~20-60ms/frame once past the 512-entry width cache), blowing the 16ms frame budget and producing the stutter + visible repaint artifacts.
• Per-child incremental clamp: FrameClampedContainer now clamps each child independently, keyed on the row array it returned. A stable transcript is never rescanned; a changed sibling pays only for its own rows. Frame-relative overflow indexes are preserved.
• Identity-stable wrappers: LocalActivityRegion / StableSlotHost / GutterComponent / BannerComponent reuse their row arrays while children and width are unchanged; TranscriptModelComponent returns a fresh array per live update instead of mutating a reused frame buffer (same-identity mutation is now invisible to the clamp).
• Surface layout memoization: SurfaceManager.layout()/linearLayout() memoize per viewport until registry, user-state, focus, activation, or collapse flags change; setCollapsed invalidates on flag flips so collapse hysteresis stays identical to fresh computation. The dock's surface budget reuses lane row counts via measureRows() instead of rendering panes a second time.
• Scroll-only doRender() on the real composition drops from ~20-60ms to <1ms on synthetic 1.5k-4k-row transcripts; remaining per-frame cost is pi-tui's viewport diff write.

Test plan

• pnpm run verify:changed — related specs, 100% coverage on touched files
• pnpm run verify:full — 3378 tests, typecheck, lint, build, happy smoke
• pnpm run check:pack — all 3 tarballs clean
• pnpm run smoke:pty (40x24 narrow PTY session) and pnpm run smoke:pty:mouse (wheel path)
• Human acceptance on dsh --profile mayfly-scroll-perf: long-transcript wheel/PgUp scrolling, live streaming at tail, manual scroll away + End follow, narrow-width collapse, /new + session-switch lifecycle

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