engram tiered storage M1: on-disk paged store format + round-trip tests

Self-contained paged store (lang/runtime/engram_store.{c,h}): 16KiB slotted pages,
u32 TLV self-describing records (forward-compatible), overflow chains, B+-tree
id-index + from/to adjacency, page free-list, tombstones, double superblock + crc
recovery. Not yet wired to activation (M3). 33/33 tests pass (ASan/UBSan clean);
5k nodes/20k edges round-trip bit-exact incl 768xf32 emb + hebb; store 25MB vs 64MB
JSON. Format is final — see design §2.4.

engram/test/run_store_tests.sh

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+# M1 paged-store gate. Pure C (NOT elb/elc). Writes only under /tmp.
+set -e
+HERE="$(cd "$(dirname "$0")" && pwd)"
+SRC="$HERE/../../lang/runtime/engram_store.c"
+BIN="/tmp/test_store.$$"
+echo "compiling: gcc test_store.c engram_store.c"
+gcc -O2 -Wall -Wextra -std=c11 "$HERE/test_store.c" "$SRC" -o "$BIN"
+"$BIN"
+rc=$?
+rm -f "$BIN"
+rm -rf /tmp/engram-store-test-*
+exit $rc