feat(runtime): verify offline access tokens and DPoP proofs

[DanielJD1216]

Summary

• add dependency-free offline Ed25519 access-token and DPoP verification
• enforce strict compact JOSE, recursive duplicate-member rejection, exact EdDSA headers and claims, bounded trusted issuer keys, and canonical RFC 7638 sender thumbprints
• normalize verified identity, request, nonce, replay, and audience bindings into the existing frozen durable transport context
• extend PostgreSQL Story 2 with a real signed-pair path through durable authorization, protected retrieval, release-time reauthorization, exact receipt consumption, buffer clearing, pool recreation, and replay denial
• keep the verifier disconnected from public exports, listeners, MCP, CLI, live identity providers, private data, and production paths

Security boundary

This PR remains a synthetic Gate B slice. It does not add OAuth/OIDC discovery, network JWKS fetches, live connectors, listener composition, private-data processing, deployment, or production-test authorization.

Invalid credentials collapse to the existing generic credential_invalid 401 response. Raw tokens, proofs, nonces, replay IDs, and sender thumbprints are not written to Story 2 evidence.

Verification

• focused Gate B tests: 43/43
• complete Alpha test suite: 196/196
• PostgreSQL 16 Story 2 conformance: 44/44
• Gate B scope baseline and mutation suite: pass
• owner issue boundary: pass
• publish:readiness: pass
• safety findings: 0
• public claim findings: 0
• immutable cryptographic review: PASS
• immutable TDD/scope/documentation review: PASS

Reviewed source candidate before commit:

base:   ef06137db60db5b1b3d43b5a17a604be872f2483
digest: e6993fd9fff45d90c032c47b7e0d123189f262641d2149188c2f710d4fd45e4e
files:  10

Closes #292