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

src/shared/protocol.h

@@ -151,8 +151,9 @@ bool protocol_send_str(ProtocolSession* session, const char* data);
 char* protocol_receive_str(ProtocolSession* session);
 /* Redacted string variants: identical wire framing to protocol_send_str /
  * protocol_receive_str, but the payload body is replaced by `<redacted>` in the
- * LOG_DEBUG_PROTO debug log.  Used for secrets (daemon auth username/digest) so
- * a --verbose log can never capture a replayable credential. */
+ * LOG_DEBUG_PROTO debug log.  Used for daemon auth material (the username and
+ * the proof/signature fields) so a --verbose log can never capture a credential
+ * that could be replayed. */
 bool protocol_send_str_redacted(ProtocolSession* session, const char* data);
 char* protocol_receive_str_redacted(ProtocolSession* session);
 bool protocol_send_data(ProtocolSession* session, const Data* data);