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).

memory.el

@@ -43,8 +43,32 @@ fn mem_strengthen(node_id: String) -> Void {
     engram_strengthen(node_id)
 }
 
+// mem_tombstone — immutable "delete": KEEP the node and all its edges; record a
+// Tombstone marker (content = target id, label "tombstone:<id>", wired with a
+// "tombstones" edge). Never engram_forget. Default bounded list reads hide
+// tombstoned nodes; ?include_deleted=1 recovers them. This is the ONE canonical
+// tombstone helper — every forget path routes through it. Defined here in
+// memory.el (imported first) so awareness.el and neuron-api.el can both call it.
+fn mem_tombstone(node_id: String) -> String {
+    let tags: String = "[\"Tombstone\",\"status:deleted\"]"
+    let marker: String = engram_node_full(
+        node_id, "Tombstone", "tombstone:" + node_id,
+        el_from_float(0.01), el_from_float(0.01), el_from_float(1.0),
+        "Episodic", tags)
+    if !str_eq(marker, "") {
+        engram_connect(marker, node_id, el_from_float(1.0), "tombstones")
+    }
+    return marker
+}
+
+// mem_forget — NOTE: no longer a hard delete. Engram nodes are immutable, so
+// this now TOMBSTONES (via mem_tombstone): the node and its edges are kept and
+// stay recoverable. Every caller (the /memory/forget route and the cultivate
+// forget op) is non-destructive as a result. Internal GC that genuinely needs
+// removal (session-summary replace, telemetry pruning) calls engram_forget
+// directly and is unaffected by this.
 fn mem_forget(node_id: String) -> Void {
-    engram_forget(node_id)
+    let _marker: String = mem_tombstone(node_id)
 }
 
 // mem_consolidate — structural scan plus salience-evolution pass.