chore: clean analyzer to zero issues

Whole-project `flutter analyze` exited 1 on 17 pre-existing info/warning
lints (no errors) in dev tools and test files. Clean them so analyze is
green:
- editor_repository_test: drop the redundant sqflite_common import; keep
  the used utils import with a transitive-dep ignore.
- pen_* widget tests: `(_, __)` wildcard params -> `(_, _)`.
- search_snippet_test / gen_bench_pdf / gen_dense_strokes: drop needless
  interpolation braces; remove an unused `Size` show and an unused local;
  mark the gen tool as print-allowed.

No runtime behavior changed. flutter analyze: No issues found. Affected
tests: 22/22 pass.

test/editor_repository_test.dart

@@ -9,8 +9,7 @@ import 'dart:io';
 
 import 'package:flutter_test/flutter_test.dart';
 import 'package:path/path.dart' as p;
-import 'package:sqflite_common/sqlite_api.dart';
-import 'package:sqflite_common/utils/utils.dart' as sqflite_utils;
+import 'package:sqflite_common/utils/utils.dart' as sqflite_utils; // ignore: depend_on_referenced_packages
 import 'package:sqflite_common_ffi/sqflite_ffi.dart';
 
 import 'package:badnote/editor/engine/stroke_model.dart';

test/pen_canvas_widget_test.dart

@@ -36,7 +36,7 @@ void main() {
               strokeWidth: 0.004,
               allowFingerDrawing: allowFinger,
               onStrokeComplete: onComplete,
-              onEraseStroke: (_, __) {},
+              onEraseStroke: (_, _) {},
             ),
           ),
         ),

test/pen_coordinate_widget_test.dart

@@ -34,7 +34,7 @@ void main() {
               color: const Color(0xFF000000),
               strokeWidth: 0.004,
               onStrokeComplete: committed.add,
-              onEraseStroke: (_, __) {},
+              onEraseStroke: (_, _) {},
             ),
           ),
         ),

test/pen_tool_widget_test.dart

@@ -34,7 +34,7 @@ void main() {
                 strokeWidth: 0.02,
                 allowFingerDrawing: allowFinger,
                 onStrokeComplete: onComplete,
-                onEraseStroke: (_, __) {},
+                onEraseStroke: (_, _) {},
               ),
             ),
           ),

test/pen_zoom_widget_test.dart

@@ -29,7 +29,7 @@ void main() {
                 color: const Color(0xFF000000),
                 strokeWidth: 0.004,
                 onStrokeComplete: (_) {},
-                onEraseStroke: (_, __) {},
+                onEraseStroke: (_, _) {},
               ),
             ),
           ),

test/search_snippet_test.dart

@@ -53,7 +53,7 @@ void main() {
 
   test('the full match is included even when longer than the window', () {
     final long = 'x' * 100;
-    final s = snippetFor('pre ${long} post', long, window: 10)!;
+    final s = snippetFor('pre $long post', long, window: 10)!;
     expect(s.match, long);
     expect(s.matchLength, 100);
   });