engram tiered storage M3.5: persist activation field updates (pre-flip gate)

Flag-on checkpoint now full-walks the resident graph: store_put_node (WM weight,
activation_count, last_activated, wm_anchor) + store_put_edge (hebb, last_fired)
for every node/edge, then engram_checkpoint. Uses store_put_edge (idempotent
upsert) not store_hebb_batch, because activation FORMS new hebbian-associate edges
that bypass the create hook and delta-only hebb_batch can't create them. Store-on
boot now applies the same WM-halving + floor + cap transforms as engram_load.

This is the hebb-survives-restart fix. Gate: reboot from neuron.egm with
snapshot.json deleted -> edge hebb + activation_count survive unchanged, WM weight
survives with identical boot transform; negative control proves persist is
load-bearing (hebb->0 without it). M1 33/33 + M2 36/36 + M3 parity PASS, ASan/UBSan
clean, flag-off untouched. Engine unchanged (boundary held).

engram/test/test_m3_parity.c

@@ -115,12 +115,19 @@ int main(int argc, char** argv){
         if (!engram_store_boot(S(dir))){ fprintf(stderr, "store boot failed\n"); return 2; }
         snprintf(p, sizeof p, "%s/on_graph.json", dir);
         engram_save(S(p));                         /* export resident (== store) */
+        /* Checkpoint the freshly-imported (pristine) graph — this is the state
+         * the reboot comparison expects to round-trip. Under M3.5 a checkpoint
+         * persists the resident graph's CURRENT field state, so it must run
+         * BEFORE activation mutates fields in place; activation itself is
+         * exercised below only for the activation-result-set parity check. The
+         * M3.5 gate (test_m35_hebb_persist) separately proves that a checkpoint
+         * taken AFTER activation durably carries the learned hebb/WM state. */
+        engram_store_checkpoint();
         el_val_t act = engram_activate_json(S(QUERY), (el_val_t)3);
         snprintf(p, sizeof p, "%s/on_act.json", dir);
         write_file(p, EL_CSTR(act));
         printf("[on] nodes=%lld edges=%lld\n",
                (long long)(int64_t)engram_node_count(), (long long)(int64_t)engram_edge_count());
-        engram_store_checkpoint();
         engram_store_close();
         return 0;
     }