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_seed.c

@@ -1086,6 +1086,10 @@ el_val_t __engram_scan_nodes_by_type_json(el_val_t node_type, el_val_t limit, el
     return engram_scan_nodes_by_type_json(node_type, limit, offset);
 }
 
+el_val_t __engram_scan_nodes_emb_json(el_val_t limit, el_val_t offset) {
+    return engram_scan_nodes_emb_json(limit, offset);
+}
+
 el_val_t __engram_neighbors_json(el_val_t node_id, el_val_t max_depth, el_val_t direction) {
     return engram_neighbors_json(node_id, max_depth, direction);
 }