Tombstone the remaining forget paths (close the immutability arc)

Phase 1 tombstoned memory/delete + node/delete, but the generic forget
path was still destructive. This routes every forget through the same
tombstone semantics so nothing in the daemon can hard-delete an engram
node anymore.

Canonical helper: mem_tombstone (memory.el, imported first so every module
can call it) — keep the node + its edges, record a Tombstone marker, never
engram_forget. neuron-api's tombstone_node now delegates to it (single
source of truth).

Per-path before -> after:
- memory.el `mem_forget`      hard delete (engram_forget) -> tombstone. This
  alone fixes both callers: the /api/neuron/memory/forget route
  (handle_api_forget) and the cultivate op=="forget".
- awareness.el autonomous `forget` action  engram_forget -> mem_tombstone.
  The soul can no longer autonomously hard-delete a memory.
- mcp-wrapper tool_forget  was a FAKE no-op that returned {"ok","deleted"}
  without deleting OR tombstoning -> now routes to the soul's tombstoning
  /api/neuron/memory/delete; tool description fixed to say it
  supersedes/tombstones (recoverable), not "Remove a node".

Left intentionally as hard deletes (internal GC / lifecycle, not user
memory, all call engram_forget directly): session-summary replace
(chat.el), mem_consolidate dedup (memory.el), session-start telemetry
pruning (soul.el), session lifecycle (sessions.el).

Regenerated both ship paths under a 3GB physical-RSS watchdog (peak ~32MB):
dist/soul.c (single-TU amalgamation, macOS/Linux) and the per-module
dist/{memory,awareness,neuron-api}.c (Windows/Linux build). Verified: no
forget handler calls engram_forget; a forget leaves the node present +
tombstone marker. Gate (neuron-ui verify-soul-contract.sh, new memory-forget
row): PRESENCE + IMMUTABILITY PASS.

awareness.el

@@ -446,8 +446,10 @@ fn respond(action_json: String) -> String {
     }
 
     if str_eq(kind, "forget") {
-        engram_forget(payload)
-        return "{\"outcome\":\"forgotten\",\"id\":\"" + payload + "\"}"
+        // The soul must NOT be able to autonomously hard-delete a memory.
+        // Tombstone instead (keep node + edges, recoverable).
+        let _marker: String = mem_tombstone(payload)
+        return "{\"outcome\":\"tombstoned\",\"id\":\"" + payload + "\"}"
     }
 
     return "{\"outcome\":\"noop\"}"