fix: downgrade negentropy timeout warning to debug level

Negentropy diff timeouts are expected when relays don't support NIP-77.
The relay responds with NOTICE 'unknown envelope label' and the timeout
is hit before we recognize this is unsupported rather than a failure.

Changes:
- Downgrade from warn! to debug! in negentropy_sync_filter()
  (src/sync/relay_connection.rs:493)
- Add comment explaining timeouts are common for non-NIP-77 relays
- Update message to clarify timeout typically means no NIP-77 support

The existing fallback mechanism (lines 505-509) properly handles this
case and logs a one-time warning about falling back to REQ+EOSE.

Discovered via production sync testing against wss://git.shakespeare.diy

src/sync/relay_connection.rs

@@ -486,11 +486,12 @@ impl RelayConnection {
                 );
 
                 // Check for any failures
+                // Note: Timeouts are common for relays without NIP-77 support
                 if !output.failed.is_empty() {
-                    tracing::warn!(
+                    tracing::debug!(
                         relay = %self.url,
                         failures = ?output.failed,
-                        "Some relays failed during negentropy diff"
+                        "Negentropy diff had failures (timeout usually means relay doesn't support NIP-77)"
                     );
                 }