remove dead tertiary-tap surface (T-95)

The middle-click ("tertiary tap") path in TerminalGestureHandler was
wired wrong: build() bound onTertiaryTapDown to the secondary state
method, so a middle-click fired as if it were a right-click. The
state's onTertiaryTapDown/Up methods were unreachable, and the
onTertiaryTapUp body had a copy-paste bug (button=right instead of
middle). No production caller passed onTertiaryTapDown / onTertiaryTapUp
through, and TerminalView didn't expose them either, so the public
parameters were dead too.

Drops both layers of dead surface — option B of T-95. Same shape as
T-93's resolution (delete unused, restore later when a real consumer
needs it). Also collapses the unreachable onDragStart selectWord
branch (PanGestureRecognizer is mouse-only, so the touch path can't
fire) into a single selectCharacters call with a comment.

Companion: refines the reflow-padding test in coverage_trivials_test
to use narrow→wide reflow setup (more honest about intent, also
actually exercises the padding branch — Buffer.resize now 100%) and
clears two unnecessary_import warnings surfaced by the deletion.

Coverage: gesture_handler 55/59 -> 59/59; gesture_detector 50/50;
buffer/buffer 260/261 -> 261/261.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/src/terminal/src/ui/gesture/gesture_detector.dart

@@ -13,8 +13,6 @@ class TerminalGestureDetector extends StatefulWidget {
     this.onTapDown,
     this.onSecondaryTapDown,
     this.onSecondaryTapUp,
-    this.onTertiaryTapDown,
-    this.onTertiaryTapUp,
     this.onLongPressStart,
     this.onLongPressMoveUpdate,
     this.onLongPressUp,
@@ -35,10 +33,6 @@ class TerminalGestureDetector extends StatefulWidget {
 
   final GestureTapDownCallback? onDoubleTapDown;
 
-  final GestureTapDownCallback? onTertiaryTapDown;
-
-  final GestureTapUpCallback? onTertiaryTapUp;
-
   final GestureLongPressStartCallback? onLongPressStart;
 
   final GestureLongPressMoveUpdateCallback? onLongPressMoveUpdate;
@@ -112,9 +106,7 @@ class _TerminalGestureDetectorState extends State<TerminalGestureDetector> {
           ..onTapDown = _handleTapDown
           ..onTapUp = _handleTapUp
           ..onSecondaryTapDown = widget.onSecondaryTapDown
-          ..onSecondaryTapUp = widget.onSecondaryTapUp
-          ..onTertiaryTapDown = widget.onTertiaryTapDown
-          ..onTertiaryTapUp = widget.onTertiaryTapUp;
+          ..onSecondaryTapUp = widget.onSecondaryTapUp;
       },
     );
 

lib/src/terminal/src/ui/gesture/gesture_handler.dart

@@ -1,6 +1,5 @@
 // Based on xterm.dart v4.0.0 by xuty (MIT). See LICENSE in this directory.
 
-import 'package:flutter/gestures.dart';
 import 'package:flutter/widgets.dart';
 import 'package:clide/src/terminal/src/core/mouse/button.dart';
 import 'package:clide/src/terminal/src/core/mouse/button_state.dart';
@@ -20,8 +19,6 @@ class TerminalGestureHandler extends StatefulWidget {
     this.onTapDown,
     this.onSecondaryTapDown,
     this.onSecondaryTapUp,
-    this.onTertiaryTapDown,
-    this.onTertiaryTapUp,
     this.readOnly = false,
   });
 
@@ -39,10 +36,6 @@ class TerminalGestureHandler extends StatefulWidget {
 
   final GestureTapUpCallback? onSecondaryTapUp;
 
-  final GestureTapDownCallback? onTertiaryTapDown;
-
-  final GestureTapUpCallback? onTertiaryTapUp;
-
   final bool readOnly;
 
   @override
@@ -65,8 +58,6 @@ class _TerminalGestureHandlerState extends State<TerminalGestureHandler> {
       onTapDown: onTapDown,
       onSecondaryTapDown: onSecondaryTapDown,
       onSecondaryTapUp: onSecondaryTapUp,
-      onTertiaryTapDown: onSecondaryTapDown,
-      onTertiaryTapUp: onSecondaryTapUp,
       onLongPressStart: onLongPressStart,
       onLongPressMoveUpdate: onLongPressMoveUpdate,
       // onLongPressUp: onLongPressUp,
@@ -144,14 +135,6 @@ class _TerminalGestureHandlerState extends State<TerminalGestureHandler> {
     _tapUp(widget.onSecondaryTapUp, details, TerminalMouseButton.right);
   }
 
-  void onTertiaryTapDown(TapDownDetails details) {
-    _tapDown(widget.onTertiaryTapDown, details, TerminalMouseButton.middle);
-  }
-
-  void onTertiaryTapUp(TapUpDetails details) {
-    _tapUp(widget.onTertiaryTapUp, details, TerminalMouseButton.right);
-  }
-
   void onDoubleTapDown(TapDownDetails details) {
     renderTerminal.selectWord(details.localPosition);
   }
@@ -172,8 +155,10 @@ class _TerminalGestureHandlerState extends State<TerminalGestureHandler> {
 
   void onDragStart(DragStartDetails details) {
     _lastDragStartDetails = details;
-
-    details.kind == PointerDeviceKind.mouse ? renderTerminal.selectCharacters(details.localPosition) : renderTerminal.selectWord(details.localPosition);
+    // PanGestureRecognizer in TerminalGestureDetector only registers
+    // PointerDeviceKind.mouse, so we know the drag came from a mouse and
+    // characters are the right selection unit.
+    renderTerminal.selectCharacters(details.localPosition);
   }
 
   void onDragUpdate(DragUpdateDetails details) {