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>

test/terminal/coverage_trivials_test.dart

@@ -35,13 +35,13 @@ void main() {
 
   test('Buffer.resize pads reflow output up to the new height', () {
     final t = Terminal(maxLines: 200, onOutput: (_) {});
-    // Write content narrower than the new width — reflow should produce
-    // fewer lines than newHeight, triggering the pad-with-empty-lines
-    // branch in buffer.dart.
-    t.write('abc\r\n');
-    // Resize to a wide + tall viewport.
-    t.resize(40, 50, 8, 16);
-    expect(t.buffer.lines.length, greaterThanOrEqualTo(50));
+    // Setup: narrow viewport with wrapped continuation lines. Widening
+    // collapses them in reflow → output rows < newHeight → the
+    // pad-with-empty-lines branch in Buffer.resize runs.
+    t.resize(5, 30, 8, 16);
+    t.write('aaaaaaaaaaaaaaaaaaaa'); // wraps 4 times
+    t.resize(80, 30, 8, 16); // widen, same height
+    expect(t.buffer.lines.length, 30);
   });
 
   test('TerminalPainter.paintLine handles wide (CJK) cells without skipping the skip', () {

test/terminal/render_test.dart

@@ -11,7 +11,6 @@ import 'package:clide/src/terminal/src/ui/controller.dart';
 import 'package:clide/src/terminal/src/ui/cursor_type.dart';
 import 'package:clide/src/terminal/src/ui/terminal_text_style.dart';
 import 'package:clide/src/terminal/src/ui/themes.dart';
-import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';