docs: seed architecture/ with capability pages and glossary

[lesnik512]

First content for the architecture/ truth home established in #16 — documents existing behavior, no code change.

Capability pages

• container-lifecycle.md — setup_di, fetch_di_container, the lifespan reopen/close-across-cycles behavior, and build_di_container's per-connection scoped child containers (the Request → REQUEST / WebSocket → SESSION scope mapping + context injection).
• dependency-resolution.md — FromDI / Dependency: how a route or WebSocket handler declares a dependency and resolves it against the request-scoped container; provider-vs-type resolution, use_cache, and reaching ACTION scope.
• glossary.md — ubiquitous language: root vs child container, connection, scope mapping, context provider, dependency marker.
• README.md — capability index.

Notes

• Two capability files (not six per-symbol, not one mega-page) — the natural seam is lifecycle/scope wiring vs endpoint resolution.
• Documenting existing behavior, so per the convention this is a docs chore with no change bundle; future behavior changes promote into these files in the same PR.

just check-planning → planning: OK; eof/lint clean.

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