feat(layout): add overflow and scroll support to layout and renderer

[RtlZeroMemory]

Summary

• add optional container overflow props (visible | hidden | scroll) and validated scrollX/scrollY inputs for box/row/column
• extend layout metadata with scrollX, scrollY, contentWidth, contentHeight, viewportWidth, and viewportHeight
• clamp scroll offsets during layout and apply layout-time child offset shifts by (-scrollX, -scrollY)
• integrate scroll viewport clipping and scrollbar rendering (1-column right bar, 1-row bottom bar, shared corner cell) using existing scrollbar glyph helpers
• update hit-testing to account for overflow modes and scroll viewport clipping semantics
• mirror runtime collection scroll state (VirtualList/Table/Tree) into layout metadata during rendering as an incremental migration path
• document overflow/scroll behavior in docs/guide/layout.md

Tests

• add 3 new deterministic layout test files:
  • layout.overflow-hidden.test.ts (19 tests)
  • layout.overflow-scroll.test.ts (26 tests)
  • layout.scroll-hitTest.test.ts (13 tests)
• add extra overflow metadata assertions in layout.edgecases.test.ts

Validation

• npm run build
• npm run typecheck
• node scripts/run-tests.mjs (pass: 2409/2409)

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Compute scrollable extent from content origin

measureContentBounds derives contentWidth/contentHeight as max - min, and resolveOverflow later clamps scrollX/scrollY from that span; this drops the offset between the viewport origin and the first child when children are laid out away from the content origin (for example via signed margins), so the computed max scroll can be too small to ever reveal the right/bottom edge (and too large when children start negative, allowing blank overscroll). In those layouts, valid content becomes permanently unreachable even though it overflows the viewport.

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[RtlZeroMemory]

Fixed in f35ac21.

I changed measureContentBounds to compute content extents from the container content origin (cx/cy) instead of span (max - min), and updated row/column/box callers to pass the content origin. This fixes under-clamping when content starts at a positive offset and avoids blank overscroll when content starts negative.

Added regression coverage in layout.edgecases.test.ts (scroll clamp uses content-origin extent when children start after origin).

Validation run after the fix:

• npm run lint
• npm run typecheck
• node scripts/run-tests.mjs (2410/2410 pass)