feat: add VirtualTable component (virtualization-first, config-driven)

[gethinwebster]

What

Adds VirtualTable — a new, additive Cloudscape component that coexists with Table (not a rewrite of it), aimed at large and streaming datasets. This is the config-driven, generic framing (F1-A1) from an exploratory design study.

Why

Table is not virtualization-first, and several AWS consoles (notably CloudWatch Logs Insights) maintain bespoke windowed grids because of it. VirtualTable provides first-class row virtualization, arbitrary non-tabular row expansion, and a full-dataset ARIA grid contract as a first-class primitive.

Highlights

• First-class row virtualization: single owned scroll container, opt-in per-row measurement, incremental scroll-offset correction (no drift for fixed rows; no observer cost when heights are fixed).
• Arbitrary, non-tabular row expansion via a render slot with measured variable expanded-row heights and a valid grid-child disclosure a11y model (real role=row → full-width role=gridcell → labeled role=region, aria-expanded/aria-controls, Tab-in / Escape-out, excluded from arrow-grid nav).
• Full-dataset ARIA under windowing: aria-rowcount/aria-rowindex, aria-colcount/aria-colindex incl. a materialised disclosure column.
• Config-driven props API mirroring Table where it transfers; imperative ref for consumer-composed live tail (scrollToEnd/scrollToItem/isPinnedToEnd).
• CloudWatch Logs Insights demo pages for the standard, patterns, and raw/file views; unit + a11y tests; usage docs; test-utils wrapper.

Testing

• npm run build green.
• npm run lint green (0 errors).
• Unit + a11y suites green (43 tests across 4 suites).

Not in scope / human-gated

• Official Cloudscape API review and design/release sign-off.
• One non-blocking lint warning remains: the debounced live-append region uses aria-live directly rather than the internal LiveRegion component — flagged as a follow-up.
• Exploratory: this is one framing/approach cell from a design study; final API is subject to team review.