docs: add literate prose to naming-drift aliases and HITL gateway changes

Adds prose sections before every code block to satisfy the
prose-before-code discipline. Each backward-compatibility alias
(process-signal, perceive-gate, reason-gate, act-gate, inject-stimulus)
now has its own subsection explaining why it exists and what new code
should use instead.

Also:
- Fixes double #+end_src in core-loop-perceive.org
- Renames inject-stimulus → stimulus-inject in heartbeat-start and
  client-handle-connection (both already had aliases)
- Adds HITL interception prose to gateway-manager.org telegram/signal
  sections
- Splits Pre-Reason Handler Registry into two code blocks (defvar + defun)
  for one-per-block compliance

lisp/core-loop.lisp

@@ -9,10 +9,6 @@
 (defvar *heartbeat-thread* nil
   "Handle to the heartbeat thread.")
 
-;; Alias: process-signal → loop-process (backward compatibility)
-(defun process-signal (signal)
-  (loop-process signal))
-
 (defun loop-process (signal)
   "The entry point to the Metabolic Pipeline: Perceive -> Reason -> Act."
   (let ((current-signal signal))
@@ -49,6 +45,9 @@
                         (list :type :EVENT :depth (1+ depth) :meta meta
                               :payload (list :sensor :loop-error :message (format nil "~a" c) :depth depth)))))))))))
 
+(defun process-signal (signal)
+  (loop-process signal))
+
 (defvar *memory-auto-save-interval* 300)
 
 (defvar *heartbeat-save-counter* 0)
@@ -69,8 +68,8 @@
                (when (>= *heartbeat-save-counter* (/ *memory-auto-save-interval* interval))
                  (setf *heartbeat-save-counter* 0)
                  (save-memory-to-disk))
-               (inject-stimulus
-                 (list :type :EVENT :payload (list :sensor :heartbeat :unix-time (get-universal-time))))))
+               (stimulus-inject
+                  (list :type :EVENT :payload (list :sensor :heartbeat :unix-time (get-universal-time))))))
            :name "passepartout-heartbeat"))))
 
 (defvar *shutdown-save-enabled* t)