docs(arch): add ADR-050 — dynamic resource ownership for runtime-spawned resources

Writes OQ-42's five decisions into ADR format:

1. Storage: reuse the repo/adapter pattern (ADR-033, fourth instance
   alongside IdentityProvider/IdentityStore/CredentialStore). New traits:
   OwnershipProvider (sync read, hot-path) + OwnershipStore (async write,
   handler lifecycle). In-memory default; persistence adapter additive.
2. Integration: AccessControl::check consults the ownership provider
   directly (Option 2). OperationSpec gains resource_id_path (JSON pointer
   into the input). Backward-compatible — ownership=None falls back to
   the static Identity.resources path.
3. Access pattern: proxy-only. Spawner owns, proxy to share via from_call
   + forwarded_for (ADR-032), teardown revokes. No grant mechanism in
   core. Future grant is additive (new trait method), stated as
   reversal-cost classification, not deferral.
4. Four edge specifics: list = scope-gate + result-filter; teardown =
   automatic, handler-driven; fleet = per-node ownership, downstream app
   tracks 'who is this for'; composition = two orthogonal checks,
   ADR-015/022 unchanged.

Reviewed: zero critical issues. Two warnings fixed (None-handling in the
check sketch, missing ADR-004 cross-ref). One suggestion applied
('v1 mechanism' → 'initial mechanism' to avoid hedging misread).

docs/architecture/README.md

@@ -103,6 +103,7 @@ The alknet-call crate is **implemented and reviewed** — both the server-side c
 | [047](decisions/047-remove-direct-call-http-surface.md) | Remove the Direct-Call HTTP Surface; Gateway Is the Sole Invoke Path | Proposed |
 | [048](decisions/048-websocket-native-session-not-gateway.md) | WebSocket Carries the Native Call-Protocol Session, Not the Gateway Shape | Accepted |
 | [049](decisions/049-streaming-handler-for-subscriptions.md) | Streaming Handler for Subscription Operations | Accepted |
+| [050](decisions/050-dynamic-resource-ownership-for-runtime-spawned-resources.md) | Dynamic Resource Ownership for Runtime-Spawned Resources | Accepted |
 
 ## Open Questions