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.

memory.el

@@ -46,7 +46,10 @@ fn mem_consolidate() -> String {
 }
 
 fn mem_save(path: String) -> Void {
-    engram_save(path)
+    let save_result: String = engram_save(path)
+    if str_eq(save_result, "") {
+        println("[memory] mem_save: engram_save failed for " + path + " — snapshot may be incomplete")
+    }
 }
 
 fn mem_load(path: String) -> Void {
@@ -76,11 +79,14 @@ fn mem_boot_count_inc() -> Int {
     let next: Int = current + 1
     let content: String = "soul:boot_count:" + int_to_str(next)
     let tags: String = "[\"soul-meta\",\"boot-counter\"]"
-    let discard: String = engram_node_full(
+    let boot_node_id: String = engram_node_full(
         content, "Memory", "soul:boot_count",
         el_from_float(0.9), el_from_float(0.9), el_from_float(1.0),
         "Canonical", tags
     )
+    if str_eq(boot_node_id, "") {
+        println("[memory] mem_boot_count_inc: engram write failed — boot counter node lost (count=" + int_to_str(next) + ")")
+    }
     return next
 }