Add Acme-style left+middle mouse chord = Cut

Left-button chords only (right button stays the radial). With the left
button held while selecting, a middle-button tap cuts the current selection
and is consumed, so it does not also trigger a primary-selection paste;
middle-click without the left held still pastes as usual.

Implemented in the shared application event filter: track the left button
held inside the active view, and on a middle press while held trigger
edit_cut. The filter is now installed unconditionally (it serves both the
radial gesture and the chord). All 5 ctest suites green.

docs/PLAN.md

@@ -128,6 +128,16 @@ Therefore the palette is replaced, not extended, and the replacement is built on
 - Not yet done: free-form stroke gestures (only button-press gesture so far);
   user-file config loading (built-in default only); frecency.
 
+### Mouse chording (Acme-style) — DONE (first increment)
+- Left+middle chord = **Cut**: with the left button held while selecting
+  (button 1 anchor), a tap of the middle button (button 2) cuts the current
+  selection. Driven by the same application event filter — it tracks the left
+  button held inside the view and, on a middle press while held, triggers
+  `edit_cut` and consumes the event. Middle-click alone (no left held) still
+  does the usual primary-selection paste.
+- Right-button chords are intentionally **not** ported — the right button is the
+  radial gesture. (Revisit later.)
+
 ### M5 — Command vocabulary + project model + switchers — NEXT
 - `:`-verb pack via `KTextEditor::Command`: `sort`, `align`, `json`, `b64`,
   `uuid`, `case`, `pipe <shell>`, etc. These populate M-x and the radial.

src/plugin/ollieplugin.cpp

@@ -67,6 +67,11 @@ OllieView::OllieView(OlliePlugin *plugin, KTextEditor::MainWindow *mainWindow)
     }
 
     setupRadials();
+
+    // One application-wide filter serves both the mouse-button radial gesture
+    // and the Acme-style left+middle chord, catching presses no matter which
+    // internal child of the view actually receives them.
+    qApp->installEventFilter(this);
 }
 
 OllieView::~OllieView()
@@ -284,7 +289,6 @@ void OllieView::setupRadials()
     m_radialDefs = RadialConfig::builtinDefault();
 
     QWidget *w = m_mainWindow->window();
-    bool wantMouseGestures = false;
 
     for (int i = 0; i < m_radialDefs.size(); ++i) {
         const RadialDef &def = m_radialDefs.at(i);
@@ -312,16 +316,9 @@ void OllieView::setupRadials()
             }
             if (btn != Qt::NoButton) {
                 m_buttonToRadial.insert(static_cast<int>(btn), i);
-                wantMouseGestures = true;
             }
         }
     }
-
-    // A single application-wide filter catches the gesture no matter which
-    // internal child of the view actually receives the press.
-    if (wantMouseGestures) {
-        qApp->installEventFilter(this);
-    }
 }
 
 bool OllieView::eventFilter(QObject *obj, QEvent *event)
@@ -350,6 +347,31 @@ bool OllieView::eventFilter(QObject *obj, QEvent *event)
         }
     }
 
+    // Acme-style left+middle chord = Cut. Button 1 (left) held while selecting
+    // is the anchor; a tap of button 2 (middle) cuts the current selection.
+    // Right-button chords are intentionally not ported (RMB is the radial).
+    // Middle-click alone (no left held) still does the usual primary paste.
+    if (event->type() == QEvent::MouseButtonRelease) {
+        if (static_cast<QMouseEvent *>(event)->button() == Qt::LeftButton) {
+            m_leftDownInView = false;
+        }
+    } else if (event->type() == QEvent::MouseButtonPress) {
+        auto *mev = static_cast<QMouseEvent *>(event);
+        KTextEditor::View *v = m_mainWindow->activeView();
+        auto *w = qobject_cast<QWidget *>(obj);
+        const bool inView = v && w && v->isAncestorOf(w);
+
+        if (mev->button() == Qt::MiddleButton && m_leftDownInView && inView) {
+            if (QAction *cut = findActionByName(QStringLiteral("edit_cut"))) {
+                cut->trigger();
+            }
+            return true; // consume: this is the chord, not a primary paste
+        }
+        if (mev->button() == Qt::LeftButton && inView) {
+            m_leftDownInView = true; // anchor set; selection proceeds normally
+        }
+    }
+
     // Suppress the context menu that would otherwise follow an RMB gesture.
     if (event->type() == QEvent::ContextMenu
         && m_buttonToRadial.contains(static_cast<int>(Qt::RightButton))) {

src/plugin/ollieplugin.h

@@ -89,6 +89,7 @@ private:
     QList<RadialDef> m_radialDefs;
     QHash<int, int> m_buttonToRadial; // Qt::MouseButton (int) -> def index
     bool m_gestureActive = false;     // an RMB drag gesture is in progress
+    bool m_leftDownInView = false;    // LMB held inside the view (Acme chord anchor)
     QHash<QString, QPointer<QAction>> m_actionsById;
 };