fix: add 30-min wall-clock timeout to incremental convergence loop

Without a bound, the loop runs forever if sources never serve an
incremental close enough to head (e.g. full snapshot base slot is
too old). After 30 minutes, proceed with the best incremental
available or none.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/agave-container/snapshot_download.py

@@ -601,8 +601,18 @@ def download_best_snapshot(
             log.info("Rolling incremental download (base slot %d, convergence %d slots)...",
                      full_snap_slot, convergence_slots)
             prev_inc_filename: str | None = None
+            loop_start: float = time.monotonic()
+            max_convergence_time: float = 1800.0  # 30 min wall-clock limit
 
             while True:
+                if time.monotonic() - loop_start > max_convergence_time:
+                    if prev_inc_filename:
+                        log.warning("Convergence timeout (%.0fs) — using %s",
+                                    max_convergence_time, prev_inc_filename)
+                    else:
+                        log.warning("Convergence timeout (%.0fs) — no incremental downloaded",
+                                    max_convergence_time)
+                    break
                 inc_fn, inc_mirrors = probe_incremental(fast_sources, full_snap_slot)
                 if inc_fn is None:
                     if prev_inc_filename is None: