fix(yoga): resolve in-flow child percent against parent content box (B7 / #60)

[re-marked]

Closes #60.

Summary

<Box width="100%"> inside a bordered or padded parent used to overflow into the parent's border/padding zone — the percent resolved against the parent's OUTER width instead of its content-box width. A right-anchored element (e.g. a titlebar close button at x = N − 1) would land underneath the right border and become invisible or partially clipped.

Pre-fix the doc comment in `layoutNode` already asserted "Per CSS, a % width resolves against the parent's content-box width" — but the code passed outer width as the percent reference. The comment was right; the code was wrong. The fix is two lines.

What changed

`packages/shared/src/yoga-layout/index.ts` — `ownerW`/`ownerH` (the reference sizes used when resolving children's percentage values) now derive from `innerMainSize`/`innerCrossSize` (parent content box) instead of `width`/`height` (parent outer). `innerMainSize`/`innerCrossSize` were already computed two lines above.

Behavior cascade

• ✓ width="100%", height="100%", min/maxWidth: percent, min/maxHeight: percent, flexBasis: percent on an in-flow child all resolve against parent content box. Matches CSS containing-block rules.
• ✓ position: relative percent offsets resolve against parent content box.
• ⚠ Child percent margin / padding / border now also resolve against parent content box, not parent outer. Yoga upstream resolves them against outer. Documented deviation — TUI consumers expect "% on a child = fraction of available space inside the parent", which is more consistent with this fix. Rare in practice; small magnitude even when present.
• ✓ Absolute children unaffected — they continue to use the parent's padding-box per CSS §10.1 (layoutAbsoluteChild is on a separate path and untouched).
• ✓ Per-edge position values (top/right/bottom/left) all resolve against ownerWidth per existing Yoga upstream convention — not changed here.

Test plan

• failing repro test (packages/shared/src/yoga-layout/index.test.ts) — bordered parent + width="100%" child asserts content-box width
• padded parent + width="100%"
• BOTH border + padding subtracted
• vertical axis (height="100%")
• non-100% percent (width="50%" against content box)
• no-border / no-padding regression case (no behavior change when there's nothing to subtract)
• minWidth, maxWidth percent
• flexBasis percent on a row container
• position: relative left percent
• documentation test pinning the Yoga-upstream deviation for child margin percent
• documentation test pinning that position: absolute children stay on the padding-box path
• pnpm typecheck / lint / build / test — 772 passed (was 760; +12 new for B7)