docs(architecture): add ADR-025 — vault local-only dispatch, drop irpc

Drops irpc from alknet-vault entirely. The vault's dispatch is now direct
method calls on VaultServiceHandle — no VaultProtocol enum, no
VaultMessage, no VaultServiceActor, no mpsc channel, no Service trait, no
RemoteService trait, no postcard serialization. The vault is local-only by
construction.

The core security argument: irpc made the vault remote-capable by default
(RemoteService generated unless no_rpc is passed). The IrohProtocol handler
forwards all messages without auth. The docs framed 'register an ALPN' as a
server-setup change. This is the default-insecure anti-pattern — security
should be opt-in, not opt-out. ADR-025 inverts the default: local-only is
the only mode, and remote access requires building a separate vault-server
crate (a visible architectural act, not a flag flip).

The actor path was already dead code — service.md said 'prefer
VaultServiceHandle directly — no channel, no serialization.' The actor
existed only to make irpc's Service trait work, which existed only to make
RemoteService work, which was the footgun. VaultServiceHandle's
Arc<RwLock> provides concurrent reads and exclusive writes — better
throughput than the actor's sequential processing.

DerivedKey serialization simplifies: always redact on serialize (for
logging safety), reject '[REDACTED]' on deserialize with an error. No
'postcard preserves bytes' path. This resolves review #002 W8 (silent
corruption on JSON-deserialized DerivedKey).

Resolves:
- OQ-21: remote vault access — resolved (not deferred). Not a vault crate
  feature; if needed, a separate vault-server crate with its own ADR.
- C7: vault-server-crate question decided — not created now, not precluded.
- C8: operation access policy table dissolved — all operations local-only
  by default; if a vault-server crate exposes some remotely, that crate
  defines the policy.
- W8: DerivedKey JSON deserialization — resolved (reject redacted payloads).

Amends ADR-005 (irpc remains for alknet-call, not for alknet-vault),
ADR-018 (vault is even more standalone — zero RPC framework deps),
ADR-019 (vault is the only layer, not just the only direct-caller layer),
ADR-008 (vault integration point unchanged, but now local-only by
construction).

docs/architecture/decisions/019-vault-assembly-layer-only.md

@@ -131,9 +131,13 @@ that door; it simply does not open it.
   assembly layer, not just registering an operation. This is a feature:
   it forces an explicit decision about what secret material a handler needs.
 - Remote vault administration (unlock a running node's vault over the
-  network) is not supported. If needed in the future, it requires a
-  separate, heavily restricted mechanism (admin scope, mTLS-only, never
-  expose the mnemonic over an unauthenticated channel) and its own ADR.
+  network) is not supported. The vault is local-only by construction
+  (ADR-025) — no remote dispatch capability exists in the vault crate. If
+  remote vault access is needed in the future, it requires a separate
+  vault-server crate that depends on both alknet-core (for auth) and
+  alknet-vault (for the handle), with a heavily restricted mechanism
+  (admin scope, mTLS-only, never expose the mnemonic over an
+  unauthenticated channel) and its own ADR.
 
 ## Assumptions