fix(a7-auth): address SCRAM auth review findings A-G

- tests: pass CREDENTIAL_KEY_LEN to unhex for the 32-byte KAT proof/sig
  (sizeof(expect) is 348, over-reading the 65-byte hex literal under ASan)
- credentials: close the username-enumeration oracle with a store-wide
  dummy_key and a deterministic per-username dummy salt; make the store's
  iteration count uniform (reject intra-file and layered disagreements) and
  answer a miss with the store-wide count; run the constant-time key compare
  even when found=false and fold the decision with bitwise AND
- credentials_compute_keys: enforce [CREDENTIAL_MIN_ITERS, CREDENTIAL_MAX_ITERS]
- tests: recompute the whole KAT independently at CREDENTIAL_DEFAULT_ITERS
  (600000) and pin the golden store line; add non-uniform-store rejection,
  bound and deterministic-dummy-salt assertions
- server: send exactly one generic STATUS_AUTH_FAILED on every failure path
  (including credentials_get_verifier failure); route all handshake exits
  through one burn path
- credentials/server: burn the base64 decoders' scratch on error, the
  hash_store_line base64/line buffers on failure, and all handshake key/proof
  material
- fuzz: guard the auth-offset scan against size_t underflow and use a found flag
- docs: drop stale digest wording, use CREDENTIAL_MIN_ITERS as the --iterations
  bound, document 0600 output for --hash-credentials (plus a stderr warning on
  a group/other-accessible stdout file), and describe the deterministic dummy
  salt in the no-oracle claims

README.md

@@ -516,9 +516,13 @@ Client and server versions must currently match exactly.
 
 Daemon modules that declare `auth users` authenticate with a SCRAM-SHA-256-style
 challenge/response against a salted PBKDF2 verifier store: no password and no
-replayable bearer credential crosses the wire or is stored on the daemon. Store
-lines are generated with `fastsync-server --hash-credentials <plaintext-file>`
-(see `RSYNC_COMPAT.md`); legacy `user:SHA256HEX` stores are rejected.
+replayable bearer credential crosses the wire or is stored on the daemon. All
+store entries share one iteration count, and an unknown user is answered with a
+deterministic per-username dummy challenge, so probing the daemon cannot
+enumerate users. Store lines are generated with
+`fastsync-server --hash-credentials <plaintext-file>` (see `RSYNC_COMPAT.md`);
+redirect that output to an owner-only (mode 0600) file, and note that legacy
+`user:SHA256HEX` stores are rejected.
 
 TLS provides encrypted TCP transport. Supplying `--ca` enables certificate
 verification; without it, traffic is encrypted but peer identity is not