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/service.md

@@ -74,15 +74,23 @@ produce different seeds.
 ### unlock_new(word_count) → phrase
 
 ```rust
-pub fn unlock_new(&self, word_count: usize) -> Result<String, VaultServiceError>;
+pub fn unlock_new(&self, word_count: usize) -> Result<Zeroizing<String>, VaultServiceError>;
 ```
 
-Generate a new random mnemonic, unlock with it, and return the phrase.
-Store the returned phrase securely — it is the root of trust. Supported
-word counts: 12, 15, 18, 21, 24.
+Generate a new random mnemonic, unlock with it, and return the phrase as
+a `Zeroizing<String>`. The returned phrase is the root of trust — it is
+heap-allocated and zeroized on drop, so it does not linger in freed
+memory. The caller should extract the phrase for secure storage (write
+down, display to user) and let the `Zeroizing<String>` drop when done.
+Do not clone the returned value or store it in a non-zeroizing container.
+Supported word counts: 12, 15, 18, 21, 24.
 
 This is the "first run" path — a new node generates its mnemonic, writes
-it down, and the vault is unlocked for the process lifetime.
+it down, and the vault is unlocked for the process lifetime. The
+`Zeroizing<String>` wrapper (from the `zeroize` crate) ensures the
+mnemonic is wiped from memory once the caller is done with it, matching
+the `Mnemonic` type's own `ZeroizeOnDrop` behavior. This resolves review
+#002 W7.
 
 ### lock()