docs(architecture): resolve review #002 remaining Tier 4 findings

Add ADR-026 (vault key model — HD derivation) recording the foundational
HD-derivation decision, 74' coin type reservation, SLIP-0010/Ed25519
default, secp256k1 feature-gating, and AES-256-GCM cipher choice. These
were previously inline rationale with no ADR (W9).

Extend ADR-018 with an explicit EncryptedData wire format lock — fields,
encoding, and semantics are frozen; no removal without a format-version
migration (W10).

Resolve the remaining guard clauses and spec decisions:

- W2: Capabilities must be immutable after construction (no interior
  mutability). Makes the Arc vs deep-copy clone semantics genuinely
  two-way.
- W5: Published to_* specs are compatibility contracts — best-effort
  mappings are two-way before first publication, one-way after. Version
  generated specs.
- W6: Salt field clarification — v2 salt is permanently unused; a future
  KDF is a different derivation family, not a version-indexed path; the
  field saves a wire-format change only.
- W7: unlock_new returns Zeroizing<String> — the mnemonic is the root of
  trust and must not linger in freed memory.
- W17: OQ-09 WASM — server-side dispatch door is honestly closed
  (Connection is concrete, tokio-bound), not implicitly preserved.
- W18: OQ-10 git — composability fork (raw smart protocol vs call-protocol
  projection) is a separate decision from ERC721 scope.
- W20: from_openapi must prefix imported error codes (HTTP_404) to avoid
  collision with protocol-level codes (NOT_FOUND). Normative rule, not
  naming convention.
- W21: ScopedOperationEnv field is private — construction via new()/
  empty(), query via allows(). Makes the future subgraph refactor
  non-breaking.
- C13: Connection::set_identity — the endpoint does not read identity()
  after handle() returns (Connection is moved into the spawned task).
  Observability is handler-side logging. Simplest honest answer.
- W1: OperationAdapter trait is async, returns Vec<HandlerRegistration>.
  from_call requires async discovery; ADR-022 changed the return type.
- W11: CompositionAuthority::as_identity() defined — constructs a
  synthetic Identity (label as id, scopes, resources) not resolvable via
  IdentityProvider. Second Identity construction path, acknowledged.
- W14: SecretKey is iroh::SecretKey (Ed25519) — consistent with the
  endpoint's iroh dependency.
- W19: Grandchild abort propagation is inherit-by-default (option a) —
  invoke() with no explicit policy inherits parent's policy. ContinueRunning
  auto-propagates to grandchildren unless explicitly overridden.

docs/architecture/decisions/023-operation-error-schemas.md

@@ -238,9 +238,23 @@ accordingly.
 
 ```rust
 // OpenAPI: 404: { schema: NotFoundError }
-// → ErrorDefinition { code: "NOT_FOUND", http_status: Some(404), schema: NotFoundError }
+// → ErrorDefinition { code: "HTTP_404", http_status: Some(404), schema: NotFoundError }
 ```
 
+**Normative rule (review #002 W20)**: `from_openapi` must not produce error
+codes that collide with the five protocol-level codes (`NOT_FOUND`,
+`FORBIDDEN`, `INVALID_INPUT`, `INTERNAL`, `TIMEOUT`). The adapter prefixes
+imported error codes with `HTTP_` and the status number (e.g., `HTTP_404`,
+`HTTP_429`) to avoid collision. This is a requirement for the adapter, not
+a naming convention — the `from_openapi` example above was previously shown
+producing `NOT_FOUND` from a 404, which collided with the protocol-level
+`NOT_FOUND` (operation not registered). The `details` field disambiguates
+in practice (present for operation-level, absent for protocol-level), but
+ADR-023 says "clients should switch on `code`, not parse `message`" — so
+the `code` alone must be unambiguous. Operations that hand-write their own
+`ErrorDefinition`s should use domain-specific codes (`FILE_NOT_FOUND`,
+`RATE_LIMITED`) rather than reusing protocol codes.
+
 The adapter maps the OpenAPI error schema to alknet's JSON Schema format
 (same conversion as input/output schemas). The `http_status` field records
 the original status code so `to_openapi` can project it back.