Steps 263-265: Phase 9e — save buffer to disk, undo/redo, integration tests

Step 263: saveBuffer/saveAllBuffers RPC methods write editBuf to disk with
workspace path resolution and parent dir creation. 12/12 tests.

Step 264: State-based undo/redo via HeadlessUndoStack. Snapshots recorded
automatically before mutations; undo/redo restore AST + editBuf. 12/12 tests.

Step 265: Phase 9e integration tests — save after undo/redo, mixed undo
states across buffers, full open→mutate→diagnose→undo→redo→save. 8/8 tests.

Phase 9e complete: 32/32 tests. Sprint 9 total: 244/244 across 5 phases.
34 MCP tools.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

editor/src/MCPServer.h

@@ -879,6 +879,61 @@ private:
             };
     }
 
+    // ---------------------------------------------------------------
+    //  Save and undo/redo tools
+    // ---------------------------------------------------------------
+    void registerSaveUndoTools() {
+        // whetstone_save_buffer
+        tools_.push_back({"whetstone_save_buffer",
+            "Save a buffer to disk. Writes the current editBuf (code "
+            "regenerated from AST) to the file path. Clears the "
+            "modified flag. Defaults to active buffer if no path given.",
+            {{"type", "object"}, {"properties", {
+                {"path", {{"type", "string"},
+                    {"description",
+                     "Buffer path to save (optional, defaults to active)"}}}
+            }}}
+        });
+        toolHandlers_["whetstone_save_buffer"] =
+            [this](const json& args) {
+                return callWhetstone("saveBuffer", args);
+            };
+
+        // whetstone_save_all_buffers
+        tools_.push_back({"whetstone_save_all_buffers",
+            "Save all modified buffers to disk. Skips unmodified "
+            "buffers. Returns count of saved and skipped files.",
+            {{"type", "object"}, {"properties", json::object()}}
+        });
+        toolHandlers_["whetstone_save_all_buffers"] =
+            [this](const json& args) {
+                return callWhetstone("saveAllBuffers", args);
+            };
+
+        // whetstone_undo
+        tools_.push_back({"whetstone_undo",
+            "Undo the last mutation on the active buffer. Restores "
+            "the previous AST and regenerated code from the snapshot "
+            "journal. Returns the new undo/redo depths.",
+            {{"type", "object"}, {"properties", json::object()}}
+        });
+        toolHandlers_["whetstone_undo"] =
+            [this](const json& args) {
+                return callWhetstone("undo", args);
+            };
+
+        // whetstone_redo
+        tools_.push_back({"whetstone_redo",
+            "Redo the last undone mutation on the active buffer. "
+            "Returns the new undo/redo depths.",
+            {{"type", "object"}, {"properties", json::object()}}
+        });
+        toolHandlers_["whetstone_redo"] =
+            [this](const json& args) {
+                return callWhetstone("redo", args);
+            };
+    }
+
     void registerWhetstoneTools() {
         registerASTTools();
         registerAnnotationTools();
@@ -886,5 +941,6 @@ private:
         registerDiagnosticTools();
         registerBatchTools();
         registerProjectTools();
+        registerSaveUndoTools();
     }
 };