docs(architecture): resolve OQ-11 and OQ-19 — all open questions resolved

OQ-11 (handler-level auth observability): Option B — handlers store
resolved identity on Connection via set_identity. Two identity scopes:
connection-level (observability, write-once-read-many) and per-request
(ACL, on OperationContext). Per-request takes precedence for ACL;
connection-level is for logging/audit only.

OQ-19 (session-scoped registries): Protocol doesn't need changes.
OperationEnv must remain a trait (not concrete) to enable session-overlay
pattern. Three-tier registry: core (static, External+Internal), session
(dynamic, Internal-only), promotion (curated review). Documented as
implementation guard in operation-registry.md.

All 19 open questions are now resolved. No open one-way or two-way doors
remain. The architecture is ready for review and implementation.

docs/architecture/crates/call/call-protocol.md

@@ -312,7 +312,7 @@ See [open-questions.md](../../open-questions.md) for full details.
 - **OQ-13** (resolved): Operation path format is `/{service}/{op}`. Remote dispatch is a separate mechanism, not a path prefix.
 - **OQ-14** (resolved): Batch is a client-side pattern of correlated `call.requested` events, not a protocol primitive.
 - **OQ-16** (resolved by ADR-014): No vault operations are exposed over the call protocol for now.
-- **OQ-19** (open): Session-scoped operation registries — agent-written operations overlaid on global registry via `OperationEnv` trait layering. Protocol doesn't need changes.
+- **OQ-19** (resolved): Session-scoped operation registries — agent-written operations overlaid on global registry via `OperationEnv` trait layering. Protocol doesn't need changes; `OperationEnv` must remain a trait.
 
 ## References