runtime: restore engram_recall_json + cgi_* accessors

neuron's soul calls engram_recall_json (neuron-api.el:618, memory.el:80) and
cgi_principal (studio.el:72). Both existed in the runtime neuron vendored
(v1.0.0-20260501) and were absent here, so the soul could not link against
current el at all.

The dangerous part is what the obvious "fix" would have done. These look like
redundant wrappers over one impl:

    engram_search_json(q, limit)  -> eg_search_json_impl(q, limit, 0)  LEXICAL
    engram_recall_json(q, limit)  -> eg_search_json_impl(q, limit, 1)  SEMANTIC

They are not interchangeable, and the split is documented at neuron-api.el:613:
search stays LEXICAL because ~40 internal call sites pass a KEY and seven of
them DELETE every record returned. Point those at a semantic matcher and they
delete fuzzy matches. Conversely, pointing recall at search silently downgrades
the mind's entire retrieval surface from semantic to lexical — no error, just
permanently worse recall.

Implemented over engram_activate(), which in this runtime already IS the
semantic path the old with_legs=1 branch built by hand (embeds the query via
eg_embed_fetch, scores by cosine, then spreads activation one hop). Output
shape matches engram_search_json — a flat array via engram_emit_node_json —
because callers parse search's shape, not activate's envelope.

Verified: neuron's soul now compiles and links against current el, boots, and
serves /health with layers initialized.

NOTE for follow-up: current el also ships engram_retrieve_geometric_json, a
structure-first retrieval that appears to be the intended successor to recall.
Repointing the two recall call sites at it may well be the right end state and
would remove the two-wrapper shape entirely — but that is a behavioral change
that must be measured against neuron/tools/retrieval-eval/'s gold set, not
assumed. This commit preserves existing behavior exactly; it does not decide
that question.

lang/runtime/el_runtime.h

@@ -1017,6 +1017,15 @@ el_val_t  stdout_to_file(el_val_t path);
 el_val_t  stdout_restore(void);
 el_val_t  el_mem_check(void);
 
+/* Semantic retrieval surface. NOT interchangeable with engram_search_json,
+ * which is lexical by design — see the note at the definition. */
+el_val_t  engram_recall_json(el_val_t query, el_val_t limit);
+
+/* CGI identity accessors (read-only). */
+el_val_t  cgi_principal(void);
+el_val_t  cgi_network(void);
+el_val_t  cgi_engram(void);
+
 #ifdef __cplusplus
 }
 #endif