feat: rolling incremental snapshot download loop

After the full snapshot downloads, continuously re-probe all fast sources
for newer incrementals until the best available is within convergence_slots
(default 500) of head. Each iteration finds the highest-slot incremental
matching our full snapshot's base slot, downloads it (replacing any previous),
and checks the gap to mainnet head.

- Extract probe_incremental() from inline re-probe code
- Add convergence_slots param to download_best_snapshot() (default 500)
- Add --convergence-slots CLI arg
- Pass SNAPSHOT_CONVERGENCE_SLOTS env var from entrypoint.py

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

scripts/agave-container/entrypoint.py

@@ -158,7 +158,8 @@ def maybe_download_snapshot(snapshots_dir: str) -> None:
     sys.path.insert(0, str(script_dir))
     from snapshot_download import download_best_snapshot
 
-    ok = download_best_snapshot(snapshots_dir)
+    convergence = int(env("SNAPSHOT_CONVERGENCE_SLOTS", "500"))
+    ok = download_best_snapshot(snapshots_dir, convergence_slots=convergence)
     if not ok:
         log.error("Snapshot download failed — starting without fresh snapshot")