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.

routes.el

@@ -274,6 +274,9 @@ fn handle_request(method: String, path: String, body: String) -> String {
             return engram_scan_nodes_json(9999, 0)
         }
         if str_eq(clean, "/api/graph/edges") {
+            // TODO(reliability #8): engram_save races with awareness loop mem_save().
+            // Both now use atomic write-to-temp+rename (el_runtime.c). Serialised
+            // by engram_global_mu. Future: add engram_edges_json() builtin.
             let snap_path: String = env("HOME") + "/.neuron/engram/snapshot.json"
             engram_save(snap_path)
             let snap: String = fs_read(snap_path)