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/awareness.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);
@@ -375,8 +376,8 @@ el_val_t respond(el_val_t action_json) {
     return el_str_concat(el_str_concat(EL_STR("{\"outcome\":\"strengthened\",\"id\":\""), payload), EL_STR("\"}"));
   }
   if (str_eq(kind, EL_STR("forget"))) {
-    engram_forget(payload);
-    return el_str_concat(el_str_concat(EL_STR("{\"outcome\":\"forgotten\",\"id\":\""), payload), EL_STR("\"}"));
+    el_val_t _marker = mem_tombstone(payload);
+    return el_str_concat(el_str_concat(EL_STR("{\"outcome\":\"tombstoned\",\"id\":\""), payload), EL_STR("\"}"));
   }
   return EL_STR("{\"outcome\":\"noop\"}");
   return 0;