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.

src/server/server_cli.h

@@ -3,6 +3,7 @@
 
 #include <stdbool.h>
 #include <stddef.h>
+#include <stdint.h>
 
 /* Parsed fastsync-server command line.  All string members are borrowed
  * pointers into the original argv (valid for the life of the argv array the
@@ -26,7 +27,13 @@ typedef struct ServerCliOptions {
   const char* config_path;      /* --config value, or NULL */
   const char* password_file;    /* --password-file value, or NULL (daemon) */
   const char* early_input_file; /* --early-input value, or NULL (daemon) */
-  const char** dparams;         /* raw --dparam override strings */
+  /* --hash-credentials=FILE: read `user:password` lines from FILE and print
+   * new-format credential-store lines to stdout, then exit.  Standalone mode
+   * (mutually exclusive with --daemon/--stdio). */
+  const char* hash_credentials_file;
+  bool hash_iterations_set; /* an explicit --iterations was given */
+  uint32_t hash_iterations; /* --iterations value (default CREDENTIAL_DEFAULT_ITERS) */
+  const char** dparams;     /* raw --dparam override strings */
   int dparam_count;
   const char* bind_address;   /* --address */
   int bind_family;            /* AF_UNSPEC / AF_INET / AF_INET6 */