Fix ptrmap data corruption with pre-initialize autovacuum database

[ddwalias]

Reproduce:

1. Use SQLite to initialize an auto-vacuum DB (to produce a live pointer-map page at page 2):

sqlite3 /tmp/av_corruption.db "PRAGMA auto_vacuum=FULL; VACUUM; IF NOT EXISTS __sim_autovacuum_seed__(id INTEGER PRIMARY KEY, value TEXT); INSERT OR IGNORE INTO __sim_autovacuum_seed__ VALUES (1,'seed');"

2. Let Turso mutate it:

tursodb /tmp/av_corruption.db "CREATE TABLE t(a);" \ for i in $(seq 1 2000); do target/debug/tursodb /tmp/av_corruption.db "INSERT INTO t VALUES ($i);"; done

3. Back in SQLite, run PRAGMA integrity_check; -> it reports Tree … Failed to read ptrmap key=….. However, Turso’s own PRAGMA integrity_check; still prints ok, so the engine doesn’t notice the damage.

Why the simulator misses it:

The deterministic runs always start from Turso-built databases, and never import a file produced by SQLite, so this path is never exercised.

Reference: #3895

[ddwalias]

The cause seem to be when Turso allocates leaf or overflow pages, it never writes the corresponding pointer‑map entries.

I attempted to fix this issue with this commit, but seems like I introduced a new bug in the process. For this PR, I will focus only on updating the simulator. The fix will be on another PR.

[penberg]

Hey @ddwalias! Good catch! I am disabling autovacuum by default in #3899. Let's have other people review this PR too, but assuming the improvement is good, this is eligible for the data corruption challenge reward, although functionality is disabled now.

[ddwalias]

I managed to make the integration test passes, however, the simulator is still failing, likely from another issue