v0.7.2: undo/redo — TDD

Operation-level memory undo/redo built on existing Merkle snapshot
infrastructure. undo-snapshot captures state before destructive tool
execution. /undo and /redo TUI commands send structured events.

- core-memory: undo-snapshot, undo, redo functions + 3 tests
- core-perceive: :undo/:redo sensor handlers
- core-act: auto-snapshot before non-read-only tools
- core-package: undo/redo symbol exports
- channel-tui-main: /undo, /redo commands + 2 tests
- Core: 73/73  TUI Main: 74/74

lisp/core-memory.lisp

@@ -151,6 +151,50 @@
           (error (c) (log-message "MEMORY WARNING - Failed to load snapshot: ~a" c)))))
   t)
 
+;; v0.7.2 — Undo/Redo
+(defvar *undo-stack* nil
+  "Ring buffer of pre-operation memory snapshots. Newest first, max 20.")
+(defvar *redo-stack* nil
+  "Stack of snapshots saved during undo for redo. Max 20.")
+
+(defun undo-snapshot ()
+  "Save current memory state to the undo stack."
+  (let ((snap (list :timestamp (get-universal-time)
+                    :data (memory-hash-table-copy *memory-store*))))
+    (push snap *undo-stack*)
+    (when (> (length *undo-stack*) 20)
+      (setf *undo-stack* (subseq *undo-stack* 0 20)))))
+
+(defun undo (&optional source)
+  "Restore memory to the most recent undo snapshot. Returns T on success, NIL if stack empty."
+  (declare (ignore source))
+  (if *undo-stack*
+      (let ((snap (pop *undo-stack*)))
+        (push (list :timestamp (get-universal-time)
+                    :data (memory-hash-table-copy *memory-store*))
+              *redo-stack*)
+        (when (> (length *redo-stack*) 20)
+          (setf *redo-stack* (subseq *redo-stack* 0 20)))
+        (setf *memory-store* (memory-hash-table-copy (getf snap :data)))
+        (log-message "UNDO: Memory restored to snapshot ~a" (getf snap :timestamp))
+        t)
+      (progn (log-message "UNDO: No snapshots to undo") nil)))
+
+(defun redo (&optional source)
+  "Restore memory to the most recent redo snapshot. Returns T on success, NIL if stack empty."
+  (declare (ignore source))
+  (if *redo-stack*
+      (let ((snap (pop *redo-stack*)))
+        (push (list :timestamp (get-universal-time)
+                    :data (memory-hash-table-copy *memory-store*))
+              *undo-stack*)
+        (when (> (length *undo-stack*) 20)
+          (setf *undo-stack* (subseq *undo-stack* 0 20)))
+        (setf *memory-store* (memory-hash-table-copy (getf snap :data)))
+        (log-message "REDO: Memory restored to snapshot ~a" (getf snap :timestamp))
+        t)
+      (progn (log-message "REDO: No snapshots to redo") nil)))
+
 (eval-when (:compile-toplevel :load-toplevel :execute)
   (ql:quickload :fiveam :silent t))
 
@@ -211,3 +255,50 @@
   (rollback-memory 0)
   (is (not (null (memory-object-get "snap-a"))))
   (is (null (memory-object-get "snap-b"))))
+
+(test test-undo-snapshot-restore
+  "Contract v0.7.2: undo-snapshot captures state, undo restores."
+  (let ((orig-store passepartout::*memory-store*)
+        (orig-undo passepartout::*undo-stack*)
+        (orig-redo passepartout::*redo-stack*))
+    (unwind-protect
+         (progn
+           (setf passepartout::*memory-store* (make-hash-table :test 'equal)
+                 passepartout::*undo-stack* nil
+                 passepartout::*redo-stack* nil)
+           (passepartout::undo-snapshot)
+           (setf (gethash "x" passepartout::*memory-store*) "hello")
+           (is (string= "hello" (gethash "x" passepartout::*memory-store*)))
+           (is (passepartout::undo))
+           (is (null (gethash "x" passepartout::*memory-store*))))
+      (setf passepartout::*memory-store* orig-store
+            passepartout::*undo-stack* orig-undo
+            passepartout::*redo-stack* orig-redo))))
+
+(test test-undo-redo-cycle
+  "Contract v0.7.2: redo restores undone state."
+  (let ((orig-store passepartout::*memory-store*)
+        (orig-undo passepartout::*undo-stack*)
+        (orig-redo passepartout::*redo-stack*))
+    (unwind-protect
+         (progn
+           (setf passepartout::*memory-store* (make-hash-table :test 'equal)
+                 passepartout::*undo-stack* nil
+                 passepartout::*redo-stack* nil)
+           (passepartout::undo-snapshot)
+           (setf (gethash "y" passepartout::*memory-store*) "world")
+           (is (passepartout::undo))
+           (is (null (gethash "y" passepartout::*memory-store*)))
+           (is (passepartout::redo))
+           (is (string= "world" (gethash "y" passepartout::*memory-store*))))
+      (setf passepartout::*memory-store* orig-store
+            passepartout::*undo-stack* orig-undo
+            passepartout::*redo-stack* orig-redo))))
+
+(test test-undo-empty-stack-nil
+  "Contract v0.7.2: undo returns nil on empty stack."
+  (let ((orig-undo passepartout::*undo-stack*))
+    (unwind-protect
+         (progn (setf passepartout::*undo-stack* nil)
+                (is (null (passepartout::undo))))
+      (setf passepartout::*undo-stack* orig-undo))))