feat(runtime): port engram_load_merge to released runtime + add missing WM headers

engram_load_merge was added to el-compiler/runtime in 35c1897 but never
ported to the released runtime used by Engram and the soul daemon.

awareness.el calls engram_load_merge in its sync refresh cycle; without
this function in lang/releases/v1.0.0-20260501/el_runtime.c the soul
daemon fails to compile.

Also adds header declarations for engram_wm_count, engram_wm_avg_weight,
engram_wm_top_json, and engram_load_merge — all four were added as
implementations (da116b2 / 35c1897) but their prototypes were missing from
el_runtime.h, causing implicit-function-declaration warnings and potential
ABI breakage on stricter compilers.

Identified during self-review 2026-06-30.

lang/releases/v1.0.0-20260501/el_runtime.h

@@ -601,6 +601,13 @@ el_val_t  engram_neighbors_json(el_val_t node_id, el_val_t max_depth, el_val_t d
 el_val_t  engram_activate_json(el_val_t query, el_val_t depth);
 el_val_t  engram_stats_json(void);
 el_val_t  engram_list_layers_json(void);
+/* Working memory introspection — count, mean weight, and top-N snapshot.
+ * Ported from el-compiler/runtime on 2026-06-30 self-review. */
+el_val_t  engram_wm_count(void);
+el_val_t  engram_wm_avg_weight(void);
+el_val_t  engram_wm_top_json(el_val_t n);
+/* Merge-load: add nodes/edges from a snapshot without resetting the store. */
+el_val_t  engram_load_merge(el_val_t path);
 /* engram_compile_layered_json — produce a prompt-ready text block split
  * into "[LAYER 0 — STRUCTURAL]" (non-suppressible layers, sacred fire)
  * and "[ENGRAM CONTEXT]" (standard suppressible layers). Returns "" if