Make engram deletes/updates/forgets immutable on the launch branch

The ship-soul builds from this branch, which has the bounded-persona floor
(#93) but never received the tombstone/supersede immutability fix (that
went to main; hotfix diverged before it). So the launch soul failed
verify-soul-contract IMMUTABILITY on the delete/update/forget routes —
they hard-removed engram nodes via engram_forget/mem_forget.

Apply the same fix, mirroring the knowledge routes' supersede pattern:
- node/update -> create new node + "supersedes" edge to the original, KEEP
  the original (no engram_forget).
- node/delete, memory/delete, memory/forget, cultivate forget, and the
  autonomous awareness forget -> TOMBSTONE via the canonical mem_tombstone
  (memory.el): keep the node + its edges, record a Tombstone marker, hide
  from default bounded list reads (?include_deleted recovers). Never
  engram_forget. The MCP forget tool now routes to the tombstoning delete
  instead of faking a delete.
Internal GC that genuinely removes transient nodes (awareness inbox-trigger
consume, consolidation dedup, session-summary replace, telemetry pruning)
still calls engram_forget directly and is unchanged.

Regenerated dist/soul.c (single-TU) + per-module dist/{memory,awareness,
neuron-api}.c from THIS branch's sources under a 3GB physical-RSS watchdog
(peak ~32MB), built against the release el_runtime (v1.0.0-20260501). The
bounded-persona floor is preserved — verified in the emitted C and the
linked binary (BOUNDED PERSONA / SOUL_PERSONA_NAME strings present).

verify-soul-contract.sh: GATE PASS — PRESENCE all 27 routes, IMMUTABILITY
5/5 KEPT (memory-update, memory-delete, node-update, node-delete,
memory-forget).

dist/memory.c

@@ -10,6 +10,7 @@ el_val_t mem_remember(el_val_t content, el_val_t tags);
 el_val_t mem_recall(el_val_t query, el_val_t depth);
 el_val_t mem_search(el_val_t query, el_val_t limit);
 el_val_t mem_strengthen(el_val_t node_id);
+el_val_t mem_tombstone(el_val_t node_id);
 el_val_t mem_forget(el_val_t node_id);
 el_val_t mem_consolidate(void);
 el_val_t mem_save(el_val_t path);
@@ -69,8 +70,18 @@ el_val_t mem_strengthen(el_val_t node_id) {
   return 0;
 }
 
+el_val_t mem_tombstone(el_val_t node_id) {
+  el_val_t tags = EL_STR("[\"Tombstone\",\"status:deleted\"]");
+  el_val_t marker = engram_node_full(node_id, EL_STR("Tombstone"), el_str_concat(EL_STR("tombstone:"), node_id), el_from_float(0.01), el_from_float(0.01), el_from_float(1.0), EL_STR("Episodic"), tags);
+  if (!str_eq(marker, EL_STR(""))) {
+    engram_connect(marker, node_id, el_from_float(1.0), EL_STR("tombstones"));
+  }
+  return marker;
+  return 0;
+}
+
 el_val_t mem_forget(el_val_t node_id) {
-  engram_forget(node_id);
+  el_val_t _marker = mem_tombstone(node_id);
   return 0;
 }