fix(infra): escape Gremlin properties and fix ENE shell escaping

- distribute_extraction.py: add gremlin_escape() for special chars,
  add --batch-size async mode with submitAsync()
- load_cornfield.py: pipe SQL via stdin to avoid shell metachar issues
- populate_ene_tables.py: same stdin pipe fix for neon SSH calls

scripts/load_cornfield.py

@@ -35,11 +35,11 @@ CONTAINER  = "arxiv-pg"
 # ── Postgres helpers ──────────────────────────────────────────────────────────
 
 def neon(sql: str, timeout: int = 60) -> str:
-    """Run SQL on neon-64gb arxiv DB, return stdout."""
+    """Run SQL on neon-64gb arxiv DB, return stdout. Uses stdin pipe to avoid shell escaping."""
     r = subprocess.run(
         ["ssh", NEON_HOST,
-         f"podman exec {CONTAINER} psql -U postgres -d arxiv -t -A -c \"{sql}\""],
-        capture_output=True, text=True, timeout=timeout,
+         f"podman exec -i {CONTAINER} psql -U postgres -d arxiv -t -A"],
+        input=sql, capture_output=True, text=True, timeout=timeout,
     )
     if r.returncode != 0:
         print(f"  NEON ERR: {r.stderr[:200]}")