Add opt-in lazy row details for Shiny

[gkroken]

R details callbacks currently run for every row while a table is constructed, including rows whose details are never
opened. This adds lazyDetails = TRUE to defer those callbacks until expansion. Existing behavior remains eager by default.

Each opening captures a fresh snapshot. Reactive changes do not recalculate it; sorting, paging, and virtual scrolling
retain it while expanded. Closing discards the result and recursively releases nested lazy tables and Shiny bindings.
Reopening runs the callback again, with the existing one-based row index and column-name arguments.

The implementation uses a session registry, batched JSON requests, and unique table/panel identities. Rerendering
invalidates old registrations even when data is unchanged. A dedicated output binding delegates HTML and dependency
rendering to Shiny, including cleanup when dependencies finish after collapse and widget initialization after an
offscreen panel returns. Existing keyboard expansion and aria-expanded are retained; loading uses a status and busy state.

Limitations:

• Requires a live Shiny session, with Shiny >= 1.5.0.
• Replace data by rerendering. updateReactable(data = ...) and JavaScript setData() reject lazy callback tables;
  state-only updates remain available.
• Server-side processing, static rendering, and caching rendered lazy widgets with bindCache() are unsupported.
• Nested lazy tables must be returned from lazy callbacks. Eager details and cells reject embedded lazy tables.
• All eligible rows have expanders, including callbacks that eventually return NULL.
• Callbacks execute synchronously in R; arbitrary observers created by application callbacks remain app-owned.

Validation:

• Full R suite: 980 assertions passed, including 51 lazy-details assertions.
• JavaScript: 455 tests and 9 snapshots passed.
• Shiny 1.5.0: 49 applicable lazy-details assertions passed; the eager-cache test requires Shiny >= 1.6.0 and is skipped.
• Chromium: three-level nesting, 36 leaf panels, reopening, and rerendering passed on Shiny 1.5.0 and 1.14.0.
• Delayed-dependency browser regressions passed for collapse cleanup, reopening, offscreen initialization, and keyboard/ARIA states.
• The 10,000-row lazy dashboard passed virtual-retention, nested-cleanup, data-rerender, and benchmark-cleanup acceptance.
• Production client/server builds, relevant ESLint and Prettier checks, and devtools::document() completed.

The small development fixtures under design/lazy-details/ cover nesting and delayed-dependency regressions and are
excluded from the built R package. The larger dashboard, benchmark, and dashboard acceptance script are excluded from
this PR. Earlier 10,000-row lazy acceptance results are supplemental review evidence; no performance claim is made here.

[gkroken]

Note: this is entirely AI-coded, but the functionality it provides is immense. I started with a wrapper around reactable that has worked well for my org, so I figured it might be of use in the base package.