fix(reliability): engram-write — guard all fire-and-forget writes

Every engram_node_full call that dropped its return value now binds it
and emits a println on empty string. engram_save calls in consolidate,
heartbeat, and dharma-room-turn are checked for failure. The two API
handlers (log_state_event, tune_config) that skipped api_persisted()
now match the read-back-after-write contract used everywhere else in
neuron-api.el.

Files changed:
- chat.el: conv_history_persist, handle_dharma_room_turn, auto_persist
- soul.el: emit_session_start_event, seed_persona_from_env HTTP check
- memory.el: mem_save, mem_boot_count_inc
- neuron-api.el: handle_api_log_state_event, handle_api_tune_config,
  handle_api_consolidate (engram_save + session summary write)
- awareness.el: ise_post local-engram fallback path

TODO comments added for non-atomic patterns (issues #12, #13) and
the missing circuit breaker (#14) — these require new primitives.

soul.el

@@ -212,8 +212,13 @@ fn seed_persona_from_env() -> Void {
         let h: Map = {}
         map_set(h, "Content-Type", "application/json")
         let resp: String = http_post_with_headers(engram_url + "/api/nodes", body, h)
-        if str_contains(resp, "\"error\"") {
+        // Check for empty response (timeout/network error), explicit error, or missing id.
+        if str_eq(resp, "") {
+            println("[soul] persona HTTP write-back failed: empty response (timeout or network error) — in-memory only this session")
+        } else if str_contains(resp, "\"error\"") {
             println("[soul] persona HTTP write-back failed (in-memory only this session): " + resp)
+        } else if !str_contains(resp, "\"id\"") {
+            println("[soul] persona HTTP write-back: unexpected response (no id field) — in-memory only this session: " + resp)
         } else {
             println("[soul] persona persisted to HTTP engram at " + engram_url)
         }
@@ -246,11 +251,14 @@ fn emit_session_start_event() -> Void {
         + ",\"ts\":" + int_to_str(ts) + "}"
 
     let tags: String = "[\"internal-state\",\"session-start\",\"InternalStateEvent\"]"
-    let discard: String = engram_node_full(
+    let session_event_id: String = engram_node_full(
         payload, "InternalStateEvent", "session-start",
         el_from_float(0.9), el_from_float(0.9), el_from_float(1.0),
         "Episodic", tags
     )
+    if str_eq(session_event_id, "") {
+        println("[soul] emit_session_start_event: engram write failed — session-start event lost")
+    }
     println("[soul] session-start event logged (boot=" + boot_num + " nodes=" + int_to_str(node_ct) + " edges=" + int_to_str(edge_ct) + ")")
 }