feat(virtual-table): headless-core + compound-skin VirtualTable (F3-A2 design-study)

[gethinwebster]

Summary

Additive, virtualization-first VirtualTable component (coexists with Table; not a rewrite). This PR is the F3-A2 cell of a VirtualTable design study: a headless useVirtualGrid core (windowing, opt-in per-row + expanded-row measurement, coherent full-dataset grid a11y, roving active-descendant focus, arbitrary non-tabular row expansion, live-append) with a thin compound Cloudscape skin (Root/Header/HeaderCell/Body/Row/Cell/ExpandedContent) that spreads the core's roles/ARIA rather than authoring its own.

What it does

• First-class row virtualization for large / streaming datasets (single owned scroll container; fixed rows pay no ResizeObserver cost; only auto rows + expanded regions are measured; anchor-based offset correction).
• Custom (arbitrary, non-tabular) row expansion as a first-class <VirtualTable.ExpandedContent> child — valid grid-child a11y (real role=row → full-width role=gridcell → labeled role=region, aria-expanded/aria-controls, Tab-in/Escape-out), measured variable expanded-row heights under windowing.
• Full-dataset aria-rowcount/aria-rowindex + materialised disclosure column + aria-colcount/aria-colindex coherence under windowing.
• Implements the CloudWatch Logs Insights standard + patterns views and the raw/file surface (see pages/virtual-table/).

Testing

• npm run build green.
• npm run lint (eslint + stylelint + prettier) green (0 errors).
• Unit + a11y suites green: 4 suites / 60 tests (axe across views, keyboard, Escape-out, sort trigger, full-dataset ARIA, live-append debounce, focus-under-recycling).

Human-gated (out of scope for this draft)

• Official Cloudscape API review + design/release sign-off.
• One eslint warning: internal aria-live node prefers the internal LiveRegion component (follow-up).
• Full-run coverage thresholds confirmed at CI.
• This is one framing/approach cell of a design study; the final API is subject to team review.

[gethinwebster]

Superseded by #4753 (dev-v3-gethinw-virtualtable-* branch): same Headless (F3-A2) design cell, reworked with Table-matched default styling + the P6 fix pass (bounded-viewport windowing, row-height clamp, disclosure glyph, focus-gated active outline). Closing this earlier design-study PR in favour of #4753.