Migrates the README's depth content into the VitePress site,
expanding each page to be useful as standalone documentation.
Stubs replaced with real content; the README becomes the source
of truth for the short version (to be trimmed in a follow-up PR
that closes the loop).

Pages filled:

- guide/getting-started — install, deploy-the-worker-bundle (with
  code-group for Vite/Next/Webpack), construct + start walkthrough,
  send/receive, two-tab verification, React quickstart, troubleshooting
- guide/architecture — primary mode diagram, fallback mode strategy
  ladder, outbox semantics, Service Worker handoff, inbound flow,
  identity, boundary section, cross-links to ADRs
- guide/gotchas — six full sections (worker name caching, delivered-
  semantics, no-replay-buffer, browser-only, mobile coverage, SW
  delivery URL) plus a bonus "drain doesn't re-fan-out on reconnect"
  section. PR cross-links for backstory
- guide/react — provider setup, useTabMesh and useTabMeshEvent in
  depth, status shape table, common patterns (connection badge,
  remote-only handling, logout)
- reference/config — required + transport + worker + persistence +
  SW + leader + per-event option sections; defaults inline; cross-
  references to gotchas where applicable; full example at the bottom
- reference/system-events — per-event documentation: hub.connected,
  hub.disconnected, transport.* (4), event.delivery.failed,
  storage.degraded; meta-field reference; wildcard subscription pattern

Verified: `pnpm docs:build` clean, biome clean. README is unchanged
in this commit — it gets trimmed in the next PR once the docs site
is live and we can link to it from the README.