feat(engram): add accumulation layer (layer 5) — new nodes default to it, not core-identity

Implements the accumulation layer from the Layered Consciousness architecture
(provisional 64/064,262) and answers the deferred design question. Per the spec
and Will's design: new user-facing nodes (memories, knowledge, conversations) are
created in an accumulation layer at the TOP of the consciousness stack — the engram
the user sees — while the layers below (safety, core-identity, domain, imprint,
suit) shape behavior but are hidden from the user.

- Adds ENGRAM_LAYER_ACCUMULATION (5) + the layer record in engram_init_layers
  (activation_priority 50, suppressible, not injectable, transparent=0).
- engram_node and engram_node_full now assign new nodes to ENGRAM_LAYER_ACCUMULATION.
- ENGRAM_LAYER_DEFAULT stays CORE_IDENTITY ON PURPOSE: it is the fallback for LEGACY
  nodes loaded from snapshots without a layer_id, so existing data (the originator
  corpus) is NEVER migrated. New-nodes-only — the immutable-originator rule.

This is the foundation for fixing the identity-bleed / customer-isolation issue
(user data was landing in Neuron's core-identity layer). The retrieval-side
provenance filter (introspection should compile from accumulation, not the
originator corpus — Persona 64/036,574) is a follow-on, pending the batch-2
Layered Consciousness + Engram spec docs for exact semantics. Compiles clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lang/el-compiler/runtime/el_runtime.c

@@ -6031,6 +6031,14 @@ void el_cgi_init(el_val_t name, el_val_t dharma_id, el_val_t principal,
 #define ENGRAM_LAYER_DOMAIN          2u
 #define ENGRAM_LAYER_IMPRINT         3u
 #define ENGRAM_LAYER_SUIT            4u
+#define ENGRAM_LAYER_ACCUMULATION    5u
+/* New user-facing nodes (memories, knowledge, conversations) are created in the
+ * accumulation layer — the top of the consciousness stack, the engram the user
+ * sees; every layer below shapes behavior but is hidden from the user (Layered
+ * Consciousness architecture, app 64/064,262). ENGRAM_LAYER_DEFAULT stays
+ * core-identity ON PURPOSE: it is the fallback home for LEGACY nodes loaded from
+ * snapshots without a layer_id, so existing data (the originator corpus) is
+ * never migrated out of its established layer. New != legacy. */
 #define ENGRAM_LAYER_DEFAULT         ENGRAM_LAYER_CORE_IDENTITY
 
 /* Pass 3 override floor. Layer 0 nodes that received any background
@@ -6208,6 +6216,20 @@ static void engram_init_layers(EngramStore* g) {
         .transparent = 0,
         .injectable = 1
     };
+    /* Layer 5 — accumulation. The TOP of the consciousness stack: the default
+     * home for all new user-facing nodes. This is the engram the user sees;
+     * every layer below shapes behavior but is hidden from the user. Not
+     * injectable — it is the persistent user accumulation, not a swappable
+     * overlay. transparent=0: its content is surfaced to introspection (it is
+     * the user's own knowledge/memory), unlike the lower behavioral layers. */
+    g->layers[g->layer_count++] = (EngramLayer){
+        .layer_id = ENGRAM_LAYER_ACCUMULATION,
+        .name = el_strdup_persist("accumulation"),
+        .activation_priority = 50,
+        .suppressible = 1,
+        .transparent = 0,
+        .injectable = 0
+    };
 }
 
 static EngramStore* engram_get(void) {
@@ -6399,7 +6421,7 @@ el_val_t engram_node(el_val_t content, el_val_t node_type, el_val_t salience) {
     n->last_activated = now;
     n->created_at = now;
     n->updated_at = now;
-    n->layer_id   = ENGRAM_LAYER_DEFAULT;
+    n->layer_id   = ENGRAM_LAYER_ACCUMULATION; /* new user-facing node → top layer */
     g->node_count++;
     return el_wrap_str(el_strdup(n->id));
 }
@@ -6435,7 +6457,7 @@ el_val_t engram_node_full(el_val_t content, el_val_t node_type, el_val_t label,
     n->last_activated = now;
     n->created_at = now;
     n->updated_at = now;
-    n->layer_id   = ENGRAM_LAYER_DEFAULT;
+    n->layer_id   = ENGRAM_LAYER_ACCUMULATION; /* new user-facing node → top layer */
     g->node_count++;
     return el_wrap_str(el_strdup(n->id));
 }