M-INTEROCEPTION P0: add read-only engram_scan_nodes_emb_json builtin

Read routes (GET /api/embeddings, /api/graph/dump) need node embedding
vectors, but every consumer emit path deliberately drops the ~5.7KB emb
vector (include_emb=0) to stay under MCP token limits. Rather than perturb
that shared path, add a dedicated additive builtin that pages nodes WITH
their dense vector, emitting id/node_type/label/created_at/emb_dim plus emb
as a JSON array whose length equals emb_dim (so a consumer can verify the
vector round-trips). Un-embedded nodes emit emb_dim:0 / emb:[]. Same
salience-sorted, transparent-layer-skipped, bounded pagination as
engram_scan_nodes_json; default page 256.

Purely additive: engram_emit_node_json and the default include_emb=0 are
untouched, so every existing endpoint is byte-identical to trunk (verified:
scan_nodes_json still carries no emb). The HTTP route wiring in server.el is
DEFERRED to cutover per the elc-drift blocker (regenerating engram/dist
diverges ~285 lines with no source change); the builtin is exercised
directly by the pure-C gate instead.

Measured on a copy: len(emb)==emb_dim for all nodes, pagination disjoint,
existing path unchanged; full 256-node x 768-dim page = 16.7 ms / 1.18 MB.
ASan+UBSan clean.

lang/runtime/el_runtime.c

@@ -11936,6 +11936,58 @@ el_val_t engram_scan_nodes_by_type_json(el_val_t type_v, el_val_t limit, el_val_
     return el_wrap_str(b.buf);
 }
 
+/* engram_scan_nodes_emb_json — READ-ONLY, ADDITIVE (M-INTEROCEPTION P0).
+ * Paginated dump of nodes WITH their dense embedding vector, for the
+ * GET /api/embeddings + /api/graph/dump read routes. Purely additive: it does
+ * NOT touch engram_emit_node_json or the default include_emb=0 anywhere. It
+ * emits a compact, self-describing record per node — id, node_type, label,
+ * created_at, emb_dim, and emb as a JSON array of %.4g floats (length ==
+ * emb_dim, so a consumer can verify the vector round-trips). Nodes without an
+ * embedding are emitted with emb_dim:0 and emb:[]. Same salience-sorted,
+ * transparent-layer-skipped pagination as engram_scan_nodes_json.
+ * Default limit is bounded (256) to keep one page's payload sane. */
+el_val_t engram_scan_nodes_emb_json(el_val_t limit, el_val_t offset) {
+    EngramStore* g = engram_get();
+    int64_t lim = (int64_t)limit; if (lim <= 0) lim = 256;
+    int64_t off = (int64_t)offset; if (off < 0) off = 0;
+    JsonBuf b; jb_init(&b);
+    jb_putc(&b, '[');
+    if (g->node_count == 0) { jb_putc(&b, ']'); return el_wrap_str(b.buf); }
+    int64_t* idx = malloc((size_t)g->node_count * sizeof(int64_t));
+    if (!idx) { jb_putc(&b, ']'); return el_wrap_str(b.buf); }
+    int64_t live = 0;
+    for (int64_t i = 0; i < g->node_count; i++) {
+        if (engram_layer_is_transparent(g->nodes[i].layer_id)) continue;
+        idx[live++] = i;
+    }
+    engram_sort_indices_by_salience(idx, live, g->nodes);
+    int64_t end = off + lim;
+    if (end > live) end = live;
+    int first = 1;
+    char tmp[80];
+    for (int64_t i = off; i < end; i++) {
+        const EngramNode* n = &g->nodes[idx[i]];
+        if (!first) jb_putc(&b, ',');
+        first = 0;
+        jb_putc(&b, '{');
+        jb_puts(&b, "\"id\":");         jb_emit_escaped(&b, n->id ? n->id : "");
+        jb_puts(&b, ",\"node_type\":"); jb_emit_escaped(&b, n->node_type ? n->node_type : "");
+        jb_puts(&b, ",\"label\":");     jb_emit_escaped(&b, n->label ? n->label : "");
+        snprintf(tmp, sizeof(tmp), ",\"created_at\":%lld", (long long)n->created_at); jb_puts(&b, tmp);
+        int32_t dim = (n->emb && n->emb_dim > 0) ? n->emb_dim : 0;
+        snprintf(tmp, sizeof(tmp), ",\"emb_dim\":%d", dim); jb_puts(&b, tmp);
+        jb_puts(&b, ",\"emb\":[");
+        for (int32_t j = 0; j < dim; j++) {
+            snprintf(tmp, sizeof(tmp), "%s%.4g", j ? "," : "", (double)n->emb[j]);
+            jb_puts(&b, tmp);
+        }
+        jb_puts(&b, "]}");
+    }
+    free(idx);
+    jb_putc(&b, ']');
+    return el_wrap_str(b.buf);
+}
+
 el_val_t engram_neighbors_json(el_val_t node_id, el_val_t max_depth, el_val_t direction) {
     /* Re-implement here directly so we serialize without going through
      * the ElList path. Walks BFS to max_depth, emits {node, edge, hops}