engram tiered storage M2: WAL + checkpoint + crash recovery + legacy import

Write-back no-steal buffer pool makes the fsync'd WAL load-bearing (M1 was
write-through). Logical WAL with record-granularity page-LSN redo idempotency.
Checkpoint = flush dirty pages, fsync store, advance last_checkpoint_lsn,
reclaim WAL prefix. One-time snapshot.json import only when store absent;
JSON never read as the ongoing store thereafter.

Gates: 33/33 M1 (no regression) + 36/36 M2 — replay parity, torn-tail fuzz
(every byte offset), checkpoint-crash at all 5 phases, torn-page+WAL redo,
legacy-import parity, hebb-survives-crash.

engram/test/run_wal_store_tests.sh

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+# M2 WAL + checkpoint + recovery gate. Pure C (NOT elb/elc). Writes only under /tmp.
+# Recovery tests use ENGRAM_WAL_SYNC=always so every WAL record is durable at crash.
+set -e
+HERE="$(cd "$(dirname "$0")" && pwd)"
+SRC="$HERE/../../lang/runtime/engram_store.c"
+BIN="/tmp/test_wal_store.$$"
+echo "compiling: gcc test_wal_store.c engram_store.c"
+gcc -O2 -Wall -Wextra -std=c11 "$HERE/test_wal_store.c" "$SRC" -o "$BIN"
+ENGRAM_WAL_SYNC=always "$BIN"
+rc=$?
+rm -f "$BIN"
+rm -rf /tmp/engram-wal-test-*
+exit $rc