fix merkle_sync: prefix serialization writes LSB bytes instead of MSB

_prefix_write was writing the least significant pb bytes of the 64-bit
prefix, but the protocol expects the most significant pb bytes (bits
63..64-pb*8). For pb<=4 these ranges are disjoint → wire always got
zeros. _prefix_read also lacked the final left-shift to align the
result at bit 63.

This caused all level>=2 merkle sync requests/batches to receive
prefix=0 instead of the real prefix, making get_items/update_bucket_hash
find zero members → handle_batch len=7 with no data → sync completed
without actually exchanging member data.

tools/chatgui/transport/member_sync.c

@@ -212,6 +212,8 @@ static int _member_get_items(void* ctx, const char* ns, uint8_t level,
     }
     sqlite3_finalize(stmt);
     *len = off;
+    DEBUG_DEBUG(DEBUG_CATEGORY_DB_SYNC, "%s: get_items ns=%s L%d/P%016llx mask=%016llx count=%u off=%zu",
+                MS_ID, ns, level, (unsigned long long)prefix, (unsigned long long)mask, *cnt, off);
     return 0;
 }
 

tools/chatgui/transport/merkle_sync.c

@@ -59,13 +59,14 @@ uint64_t merkle_sync_level_prefix(uint64_t key, uint8_t level) {
 uint8_t merkle_sync_prefix_bytes(uint8_t level) { int bits = level * 5; return (uint8_t)((bits + 7) / 8); }
 
 static void _prefix_write(uint8_t* out, uint64_t prefix, uint8_t pb) {
-    for (int i = (int)pb - 1; i >= 0; i--) out[pb - 1 - i] = (uint8_t)(prefix >> (i * 8));
+    for (int i = 0; i < (int)pb; i++)
+        out[i] = (uint8_t)(prefix >> (8 * (7 - i)));
 }
 
 static uint64_t _prefix_read(const uint8_t* in, uint8_t pb) {
     uint64_t v = 0;
     for (uint8_t i = 0; i < pb && i < 8; i++) v = (v << 8) | in[i];
-    return v;
+    return v << (8 * (8 - (int)pb));
 }
 
 static int _popcount_u32(uint32_t v) { return __builtin_popcount(v); }