fix(ha-repl-hardening): wire snapshot production path (task 3.5b)

- SEND_SNAPSHOT uses checkpoint_lsn (not committed_lsn) for file lookup
- ha_send_snapshot_to_standby stats file for actual total_size
- ha_compute_oldest_wal_lsn scans WAL dir for min segment LSN
- Removed hardcoded 131072 and oldest_wal_lsn=0 placeholders

src/controller/config_repl.c

@@ -1740,16 +1740,21 @@ int (*xce_snapshot_chunk_hook)(endpoint_t *ep, xce_request_t *req) = NULL;
 #define SNAPSHOT_CHUNK_SIZE 65536
 
 int ha_send_snapshot_to_standby(endpoint_t *ep, const char *snap_dir,
-                                 uint64_t checkpoint_lsn, uint64_t total_size)
+                                 uint64_t checkpoint_lsn)
 {
     if (!snap_dir)
         return -EINVAL;
 
-    /* Find the snapshot file by naming convention */
     char snap_path[4096];
     snprintf(snap_path, sizeof(snap_path),
              "%s/snap-%016" PRIx64 ".capnp", snap_dir, checkpoint_lsn);
 
+    /* Stat the snapshot file to get total_size */
+    struct stat st;
+    if (stat(snap_path, &st) != 0)
+        return -errno;
+    uint64_t total_size = (uint64_t)st.st_size;
+
     FILE *f = fopen(snap_path, "rb");
     if (!f)
         return -errno;
@@ -1813,3 +1818,22 @@ int ha_send_snapshot_to_standby(endpoint_t *ep, const char *snap_dir,
     fclose(f);
     return rc;
 }
+
+uint64_t ha_compute_oldest_wal_lsn(const char *wal_path)
+{
+    DIR *d = opendir(wal_path);
+    if (!d)
+        return 0;
+
+    uint64_t oldest = UINT64_MAX;
+    struct dirent *ent;
+    while ((ent = readdir(d)) != NULL) {
+        uint64_t seg_lsn = 0;
+        if (sscanf(ent->d_name, "seg-%" SCNx64 ".wal", &seg_lsn) != 1)
+            continue;
+        if (seg_lsn < oldest)
+            oldest = seg_lsn;
+    }
+    closedir(d);
+    return (oldest == UINT64_MAX) ? 0 : oldest;
+}