[ihordiachenko]

Addresses #927.

Request-context signatures previously covered the gRPC frame and timestamp, leaving the RPC URI unsigned. A malicious backend could relabel a signed request as another method whose protobuf encoding is compatible, turning it into unintended payment or configuration authorization.

The fix uses a shared, versioned payload covering the full URI, frame, and timestamp. A domain tag and explicit lengths make the encoding unambiguous. The signer also validates Configure close addresses against its network before updating the allowlist.

Design decisions and tradeoffs:

• Invalid contexts remain ignored. They authorize nothing, while unrelated signing and valid sibling contexts can proceed. Rejecting the entire HSM request would introduce a denial-of-service path.
• Legacy signatures receive no authorization fallback. This breaks signature compatibility and requires coordinated client/signer upgrades.
• Previously applied allowlists survive in signer state. Legacy cached Configure requests become inert; the create-only cache also introduces a first-Configure upgrade caveat, documented in the security reference.
• Existing permissive behavior and allowlist replacement semantics remain unchanged. Replay protection and stricter resolver-content matching are outside this change.