#000067 bench-max #5: 16 KB page_size on hydrate target DBs

The 3090 genesis bench at 28-min progress mark showed all 4 workers
sitting in jbd2_log_wait_commit / do_get_write_access — ext4
filesystem journal serializing every page-extension event across
the 4 parallel writers. SQLite synchronous=OFF and journal_mode=
MEMORY skip SQLite's own fsync/WAL costs, but file growth still
goes through ext4's journal layer.

Fix: bump SQLite page_size from default 4 KB to 16 KB on each
hydrate target. Bigger pages mean ~4× fewer page writes during
chunk-fill, which means ~4× fewer file-extension events through
ext4's journal. Direct attack on the measured bottleneck.

page_size can only be set on an EMPTY database; VACUUM finalises
the change. The unpack CLI now opens each target file raw,
PRAGMA page_size = 16384 + PRAGMA secure_delete = OFF + VACUUM,
closes, then connect() applies schema as normal on the
pre-sized empty DB. Idempotent on re-runs since hydrate dirs
are always clean.

Also sets secure_delete = OFF (default is OFF in most builds but
explicit for hydrate writers): no overwrite-with-zeros on row
deletion, less write traffic during the chunk UPDATEs that might
free overflow pages.

Expected impact: ~30-50% reduction in chunk-fill wall time vs the
killed 30-min v2 run. Combined with the prior bench-max round
(journal_mode=MEMORY, deferred FTS, batched UPDATEs) and the
upcoming fts-pack-skip-rebuild path, full genesis should land
under 30 min total instead of the 75-min v2 extrapolation.

4 cold-unpack-routed tests pass unchanged.

arborist/cli.py

@@ -2995,10 +2995,29 @@ def _cmd_cold_unpack(args: argparse.Namespace) -> int:
     # directory; --hydrate-M picks the routing modulus. Each incoming
     # row lands on shard_for_document(root, M).
     if args.hydrate_shards_dir:
+        import sqlite3
         target_dir = Path(args.hydrate_shards_dir).expanduser()
         target_dir.mkdir(parents=True, exist_ok=True)
         M = args.hydrate_M
         target_paths = [target_dir / f"{i:03d}.db" for i in range(M)]
+
+        # #000067 bench-max: bump page_size before schema applies so the
+        # corpus uses 16 KB pages instead of SQLite's 4 KB default.
+        # Bigger pages → ~4× fewer page writes during chunk-fill →
+        # ~4× fewer ext4 metadata journal events (jbd2_log_wait_commit
+        # was the measured bottleneck on the 3090). page_size CAN ONLY
+        # be set on an empty DB; VACUUM finalises it. Idempotent on
+        # re-runs since fresh hydrate dirs are clean.
+        for p in target_paths:
+            if not p.exists() or p.stat().st_size == 0:
+                pre = sqlite3.connect(str(p))
+                try:
+                    pre.execute("PRAGMA page_size = 16384")
+                    pre.execute("PRAGMA secure_delete = OFF")
+                    pre.execute("VACUUM")  # commit the page_size change
+                finally:
+                    pre.close()
+
         targets: list = []
         for p in target_paths:
             t = connect(p)