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/crates/vault/encryption.md

@@ -218,13 +218,14 @@ not part of ADR-021's rotation scheme.
 
 | Decision | ADR | Summary |
 |----------|-----|---------|
-| AES-256-GCM for credential encryption | — | Authenticated encryption, hardware-accelerated |
+| AES-256-GCM for credential encryption | [ADR-026](../../decisions/026-vault-key-model-hd-derivation.md) | Authenticated encryption, hardware-accelerated |
 | HD derivation, not PBKDF2 | [ADR-020](../../decisions/020-hd-derivation-for-encryption-keys.md) | Seed-derived key; no password; deterministic |
 | Salt unused in v2 (wire-format compat) | [ADR-020](../../decisions/020-hd-derivation-for-encryption-keys.md) | Kept for TS compat; not used in key derivation |
-| Key derived at `m/74'/2'/0'/0'` | — | Dedicated account for encryption keys |
+| Key derived at `m/74'/2'/0'/0'` | [ADR-026](../../decisions/026-vault-key-model-hd-derivation.md) | Dedicated account for encryption keys |
 | Version-indexed paths for rotation | [ADR-021](../../decisions/021-key-rotation-via-version-indexed-paths.md) | `m/74'/2'/0'/{version-2}'` |
 | Key versioning (v1=TS PBKDF2, v2=vault HD) | [ADR-020](../../decisions/020-hd-derivation-for-encryption-keys.md) | Distinguishes derivation methods |
 | All fields base64-encoded | — | JSON serialization compatibility |
+| `EncryptedData` wire format frozen | [ADR-018](../../decisions/018-vault-standalone-crate.md) | Fields, encoding, semantics locked; no removal without migration |
 
 ## Open Questions