Fix: gate stale Realm renderer generations

[ael-dev3]

Summary

This is a focused follow-up stacked on fix/realm-renderer-recovery-followup (PR #82). It completes the remaining non-terminal renderer-recovery requirements from the Realm Renderer Recovery prompt without duplicating the lifecycle and input protections already under review.

• Consolidate Realm, title, and graphics-preference capability checks behind one cached, WebGL2-only probe.
• Remove destructive WEBGL_lose_context probing; capability checks never tear down a context.
• Assign every Realm scene load a monotonic renderer generation and suppress stale readiness, recovery, and failure callbacks after a scene is retired.
• Expose active and last-successfully-rendered generations through DOM diagnostics.
• Extend the pure lifecycle reducer with stale-generation invariants and add a non-destructive probe regression test.
• Document the generation/capability contract for future renderer work.

Scope

Frontend renderer recovery only. No SpacetimeDB schema, authorization, backend, DNS, deployment, or production-world state changes.

Verification

• Full Vitest suite: 197 files / 1,758 tests passed.
• TypeScript project build passed.
• Vite production build passed.
• Runtime asset and production-output verification passed.
• git diff --check passed.

Review note

This branch intentionally targets the existing renderer follow-up branch so the work can be reviewed as a small, mergeable increment after PR #82. Base commit: dc9ac4066cb4427dce733713e01789532fb661e3.

[ael-dev3]

Superseded by #95, which consolidates renderer generation gating and its final exception-safety repairs.