engram tiered storage M3: wire store behind ENGRAM_STORE (default off) + .egm rename

Caller-side shim in el_runtime.c maps EngramNode/Edge <-> StoreNode/Edge; engine
keeps zero soul deps (libengram boundary, design §10). Flag off = today's JSON
path byte-for-byte (proven: no neuron.egm created, graph identical). Flag on =
engram_open (import snapshot.json once into neuron.egm, else WAL-replay) +
resident load; node/edge create + forget dual-write via guarded hooks. Files
renamed engram.store->neuron.egm, engram.wal->neuron.wal.

Gate: M3 parity PASS (graph on==off byte-exact modulo ordering; snapshot round-trip;
reboot-from-egm with snapshot.json deleted; activation set+sequence identical;
ASan/UBSan clean). M1 33/33 + M2 36/36 green post-rename.

Known gap (pre-flip): in-place hebb/WM/activation_count updates during activation
are not yet persisted to the store (create/connect/forget are). Must close before
live flip so learned edges survive restart.

engram/test/test_wal_store.c

@@ -6,7 +6,7 @@
  *
  * Covers §7/M2 gates:
  *   1 replay parity      — random op stream: normal-durable path == crash-recover path
- *   2 torn-tail fuzz     — truncate engram.wal at EVERY byte offset → never crash,
+ *   2 torn-tail fuzz     — truncate neuron.wal at EVERY byte offset → never crash,
  *                          recover to the last intact record (contiguous prefix)
  *   3 checkpoint-crash   — kill at each checkpoint phase → converge, no loss past fsync
  *   4 torn-page + WAL    — corrupt a store page under WAL coverage → redo re-derives
@@ -215,19 +215,19 @@ static void test_replay_parity(void){
 /* ═══════════════════════════ TEST 2 — torn-tail fuzz ═══════════════════════ */
 #define TT_NODES 14
 static void test_torn_tail(void){
-    printf("\n== torn-tail fuzz: truncate engram.wal at every byte offset ==\n");
+    printf("\n== torn-tail fuzz: truncate neuron.wal at every byte offset ==\n");
     char base[600]; mk_dir("tornbase", base, sizeof base);
     EngramPagedStore* s = engram_open(base);
     for (int i=0;i<TT_NODES;i++){ StoreNode n; gen_node(i,0,&n); store_put_node(s,&n); store_node_free(&n); }
     store__crash(s);                             /* leave store(at ckpt) + full WAL on disk */
 
-    char sp[700], wp[700]; snprintf(sp,sizeof sp,"%s/engram.store",base); snprintf(wp,sizeof wp,"%s/engram.wal",base);
+    char sp[700], wp[700]; snprintf(sp,sizeof sp,"%s/neuron.egm",base); snprintf(wp,sizeof wp,"%s/neuron.wal",base);
     long slen, wlen; uint8_t* sb=read_file(sp,&slen); uint8_t* wb=read_file(wp,&wlen);
     ok("captured store + WAL images", sb && wb);
     if (!sb || !wb) return;
 
     char work[600]; mk_dir("tornwork", work, sizeof work);
-    char wsp[700], wwp[700]; snprintf(wsp,sizeof wsp,"%s/engram.store",work); snprintf(wwp,sizeof wwp,"%s/engram.wal",work);
+    char wsp[700], wwp[700]; snprintf(wsp,sizeof wsp,"%s/neuron.egm",work); snprintf(wwp,sizeof wwp,"%s/neuron.wal",work);
 
     int crashes=0, dirty_check=0, non_prefix=0, full_recovered=0;
     for (long t=0; t<=wlen; t++){
@@ -300,7 +300,7 @@ static void test_torn_page(void){
 
     /* corrupt the highest-id NODE data page on disk (its records are post-checkpoint,
      * so the WAL still covers them). */
-    char sp[700]; snprintf(sp,sizeof sp,"%s/engram.store",dir);
+    char sp[700]; snprintf(sp,sizeof sp,"%s/neuron.egm",dir);
     long slen; uint8_t* sb=read_file(sp,&slen);
     long pages = slen/16384;
     long victim = -1;
@@ -379,8 +379,8 @@ static void test_legacy_import(void){
 
     EngramPagedStore* s = engram_open(dir);    /* store absent + snapshot present → import */
     ok("engram_open imported the snapshot", s!=NULL);
-    char sp[700]; snprintf(sp,sizeof sp,"%s/engram.store",dir); struct stat st;
-    ok("engram.store created by import", stat(sp,&st)==0);
+    char sp[700]; snprintf(sp,sizeof sp,"%s/neuron.egm",dir); struct stat st;
+    ok("neuron.egm created by import", stat(sp,&st)==0);
     if (!s) return;
 
     int nmiss=0, embmiss=0;