docs(arch): hub-wiring cluster — aggregated env, peer_operations, from_call cleanup, alknet-hub crate spec

Three ADRs addressing gaps surfaced by the first hub consumer (alkapi):

- ADR-067: Aggregated peer-env wiring — Dispatcher::with_aggregated_env hook
  so compose_root_env reads a shared PeerCompositeEnv across all calls,
  not a fresh per-call one. The hub-defining gap (alkapi OQ-08 / G.1).

- ADR-068: PeerCompositeEnv::peer_operations override — adds
  list_operation_names() to OperationEnv, overrides on OverlayOperationEnv
  and PeerCompositeEnv. Fixes services/list-peers returning empty operation
  lists for non-local peers (alkapi G.6).

- ADR-069: from_call is a manual free function, not auto-wired — reverses
  the aspirational OQ-27 resolution to match the implementation. Cleans
  the 'v1 default' hedging language in ADR-017 and client-and-adapters.md
  (alkapi G.4).

New crate spec: alknet-hub — reusable hub pattern (aggregated env,
connection lifecycle, worker supervision with backoff, service discovery).

Spec fixes: builder API drift (with_local/with_local_streaming separation),
ScopedOperationEnv → ScopedPeerEnv type name, OQ count 51→54.

Three new OQs: OQ-52 (wait_for_close), OQ-53 (backoff defaults),
OQ-54 (inbound hook placement).

docs/architecture/questions/053-backoff-config-defaults.md

@@ -0,0 +1,19 @@
+# OQ-53: BackoffConfig Default Policy
+
+- **Origin**: [crates/hub/README.md](crates/hub/README.md)
+- **Status**: open
+- **Door type**: Two-way
+- **Priority**: low
+- **Resolution**: Not yet decided.
+
+The `BackoffConfig` struct provides configurable backoff for worker
+reconnection. The default policy (1s initial, 60s max, 2x multiplier) is a
+starting point. Whether this is the right policy for production deployments
+is an operational question, not an architectural one.
+
+The `BackoffConfig` struct shape is committed; the defaults are a starting
+point that can be changed without breaking the API. The question is whether
+to change the defaults before the first release, based on operational
+experience from the alkapi deployment.
+
+- **Cross-references**: [crates/hub/README.md](crates/hub/README.md)