feat(a7): SCRAM-SHA-256 daemon auth to replace replayable digest
Replace the challenge-less static-SHA-256 daemon bearer credential with a
SCRAM-SHA-256-style challenge/response and a salted PBKDF2 verifier store.
PROTOCOL_VERSION 2.18.0 -> 2.19.0; legacy user:SHA256HEX stores hard-reject.
- credentials: b64/rand/PBKDF2/HMAC primitives, verifier store parser,
constant-time proof verify + ServerSignature, --hash-credentials helper
- config: auth block is now [present][username]; client runs the challenge
exchange; config_burn_auth wipes plaintext/derived secrets (A7-4)
- server: gate drives the challenge, dummy verifier for unknown/off-list users
- tests: independent Python KAT, replay + legacy integration tests, fuzz paths
- docs: new store format, --hash-credentials, 2.19.0 bump
TLS verification behavior (A7-3/S1) is intentionally unchanged.
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