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.h

@@ -621,6 +621,7 @@ el_val_t  engram_get_node_by_label(el_val_t label);
 el_val_t  engram_search_json(el_val_t query, el_val_t limit);
 el_val_t  engram_scan_nodes_json(el_val_t limit, el_val_t offset);
 el_val_t  engram_scan_nodes_by_type_json(el_val_t node_type, el_val_t limit, el_val_t offset);
+el_val_t  engram_scan_nodes_emb_json(el_val_t limit, el_val_t offset);
 el_val_t  engram_neighbors_json(el_val_t node_id, el_val_t max_depth, el_val_t direction);
 el_val_t  engram_activate_json(el_val_t query, el_val_t depth);
 el_val_t  engram_stats_json(void);