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/core/config.md

@@ -41,7 +41,11 @@ pub enum TlsIdentity {
     /// RFC 7250 raw Ed25519 public key.
     /// No domain, no CA, no cert renewal. Key = identity.
     /// Same model as iroh's NodeId, but for direct QUIC connections.
-    RawKey(SecretKey),
+    /// `SecretKey` is `iroh::SecretKey` (Ed25519) — re-exported from iroh,
+    /// which alknet-core already depends on (feature-gated, ADR-010). The
+    /// key can be derived from alknet-vault at the assembly layer
+    /// (endpoint.md) or generated fresh. See OQ-12, W14.
+    RawKey(iroh::SecretKey),
 
     /// Self-signed X.509 cert for development.
     /// Generated on startup, not validated by external clients.
@@ -76,7 +80,7 @@ The reference `StaticConfig` (in `alknet-main/crates/alknet-core/src/config/stat
 // P2P / key-based identity (default for most nodes)
 let p2p_config = StaticConfig {
     listen_addr: Some("0.0.0.0:4433".parse()?),
-    tls_identity: Some(TlsIdentity::RawKey(SecretKey::generate())),
+    tls_identity: Some(TlsIdentity::RawKey(iroh::SecretKey::generate())),
     iroh_relay: None,
     drain_timeout: Duration::from_secs(2),
 };