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/016-abort-cascade-for-nested-calls.md

@@ -152,10 +152,25 @@ context.env.invoke_with_policy(
 ```
 
 The child's `OperationContext` carries the policy. If the child itself
-composes grandchildren, the policy propagates unless the child explicitly
-overrides it. This is consistent with the composition authority and scoped
-env propagation in ADR-022 — the parent handler decides the child's
-runtime context, including abort policy.
+composes grandchildren, the policy **propagates by inheritance** — the
+grandchild inherits the child's policy (which was the parent's policy,
+unless the parent overrode it for the child via `invoke_with_policy`).
+`ContinueRunning` does auto-propagate to grandchildren: if a parent opts
+its child into `ContinueRunning`, and the child composes grandchildren
+without explicitly overriding, the grandchildren also get
+`ContinueRunning`. This is consistent with the composition authority and
+scoped env propagation in ADR-022 — the parent handler decides the
+child's runtime context, including abort policy, and that decision
+propagates through the composition tree by default.
+
+**Review #002 W19 resolution**: `invoke()` with no explicit policy
+argument inherits the parent's current policy (option a). It does **not**
+reset to `AbortDependents`. A handler that wants a child to reset to the
+default must explicitly call `invoke_with_policy(...,
+AbortPolicy::AbortDependents)`. This makes the propagation predictable:
+the policy I set for my child applies to my child's children unless they
+re-decide. The `invoke()` default in operation-registry.md
+(`abort_policy: parent.abort_policy.clone()`) is correct.
 
 The `OperationEnv` trait gains an optional policy parameter. The specific
 API shape (a separate `invoke_with_policy` method, a policy field on an