fix(reliability): state-management — document and partially fix concurrent state races

Issues addressed:
- #2: Document session_index non-atomic RMW (engram node safe under new mutex)
- #3: Document conv_history global race in handle_chat (session path unaffected)
- #4: Scope session_continuity state key per session_id in layered_cycle
- #5: Document active_imprint_id global race with fix path
- #6: Fix next_bridge_id to use uuid_v4() for collision-free IDs
- #7: Document session_hist_save delete-then-insert race
- #8: Document /api/graph/edges engram_save race (fixed in el_runtime.c)
- #10: Document agentic_conv_history global race in awareness loop

Issues #1 (engram_global mutex) and #8 (atomic engram_save write-to-temp+rename)
are fully fixed in el_runtime.c (committed to foundation/el repo separately).
Issue #9 skipped — already fixed in PR #31.

soul.el

@@ -288,8 +288,11 @@ fn layered_cycle(raw_input: String) -> String {
     let cont_status: String = json_get(continuity, "status")
     let cont_action: String = json_get(continuity, "action")
 
-    // Store continuity status so imprint can adjust its response register
-    state_set("session_continuity", cont_status)
+    // Store continuity status so imprint can adjust its response register.
+    // TODO(reliability #4): session_continuity is process-global; scope per session_id
+    // when available to prevent cross-session bleed under concurrent layered_cycle calls.
+    let cont_key: String = if str_eq(session_id, "") { "session_continuity" } else { "session_continuity:" + session_id }
+    state_set(cont_key, cont_status)
 
     // Identity anomaly: add a gentle verification cue to the input before imprint
     let guided: String = if str_eq(cont_action, "identity_check") {