feat(neuron-api): add identity/values write protection

Block evolve_knowledge, evolve_memory, forget, and link_entities (to_id
direction) from modifying the 15 hardcoded identity and values node IDs.
Returns HTTP 403 with a hint to use the cultivation path instead.

Add POST /api/neuron/cultivate — the bypass endpoint for intentional
cultivation sessions. Accepts { "operation": "...", ...args } and performs
the same operations without the protection check.

Add handle_api_forget and handle_api_evolve_memory as new protected-by-
default handlers, routed at /api/neuron/memory/forget and
/api/neuron/memory/evolve respectively.

Tested: 10 verification cases — 403 on all blocked targets, 200 on
non-protected nodes and FROM-direction links, cultivate bypass confirmed.

routes.el

@@ -406,12 +406,21 @@ fn handle_request(method: String, path: String, body: String) -> String {
         if str_eq(clean, "/api/neuron/memory") {
             return handle_api_remember(body)
         }
+        if str_eq(clean, "/api/neuron/memory/evolve") {
+            return handle_api_evolve_memory(body)
+        }
+        if str_eq(clean, "/api/neuron/memory/forget") {
+            return handle_api_forget(body)
+        }
         if str_eq(clean, "/api/neuron/recall") {
             return handle_api_recall(method, path, body)
         }
         if str_eq(clean, "/api/neuron/consolidate") {
             return handle_api_consolidate(body)
         }
+        if str_eq(clean, "/api/neuron/cultivate") {
+            return handle_api_cultivate(body)
+        }
         return err_404(clean)
     }