fix(suggest): break score ties by file, line and rule

The plan sorted by score alone. List.sort is not stable, so equal-score
actions came out in an order nothing guaranteed, and two runs over the same
code could hand an agent a different first action. Ties now fall back to file,
line, rule and label, both in the global order and within each file.

lib/src/suggest.dart

@@ -14,6 +14,8 @@ class FixAction {
     required this.resolves,
     required this.score,
     required this.why,
+    this.file = '',
+    this.line,
   });
 
   final String rule;
@@ -36,6 +38,10 @@ class FixAction {
   /// Human-readable breakdown of the score, so the order is not a black box.
   final String why;
 
+  /// Where the action's first finding is: the tiebreak between equal scores.
+  final String file;
+  final int? line;
+
   String get verify => 'pipe check --rule $rule';
 
   Map<String, Object?> toJson() => {
@@ -91,7 +97,7 @@ class WorkPlan {
   /// Global ordering of causes, highest impact first.
   List<FixAction> get ordered {
     final all = [for (final plan in files) ...plan.actions];
-    all.sort((a, b) => b.score.compareTo(a.score));
+    all.sort(_byImpact);
     return all;
   }
 
@@ -208,6 +214,21 @@ class WorkPlan {
   }
 }
 
+/// Highest score first, then file, line and rule. List.sort is not stable, so
+/// sorting by score alone left equal-score actions in an arbitrary order that
+/// could differ between runs - and the agent plan is read top-down.
+int _byImpact(FixAction a, FixAction b) {
+  final byScore = b.score.compareTo(a.score);
+  if (byScore != 0) return byScore;
+  final byFile = a.file.compareTo(b.file);
+  if (byFile != 0) return byFile;
+  final byLine = (a.line ?? 0).compareTo(b.line ?? 0);
+  if (byLine != 0) return byLine;
+  final byRule = a.rule.compareTo(b.rule);
+  if (byRule != 0) return byRule;
+  return a.label.compareTo(b.label);
+}
+
 /// Severity is the dominant term: it is what distinguishes a broken
 /// interaction from a tidiness issue.
 const Map<PipeSeverity, int> _severityBase = {
@@ -363,6 +384,8 @@ WorkPlan buildWorkPlan({required String root, required FindingReport report}) {
       resolves: group.length,
       score: score,
       why: _whyOf(group, score),
+      file: file,
+      line: head.line,
     );
     byFile.putIfAbsent(file, () => []).add(action);
     fileScores.update(file, (value) => value + score, ifAbsent: () => score);
@@ -370,7 +393,7 @@ WorkPlan buildWorkPlan({required String root, required FindingReport report}) {
 
   final files = <FilePlan>[];
   for (final entry in byFile.entries) {
-    final actions = entry.value..sort((a, b) => b.score.compareTo(a.score));
+    final actions = entry.value..sort(_byImpact);
     files.add(
       FilePlan(
         file: entry.key,

test/suggest_test.dart

@@ -309,4 +309,26 @@ void main() {
     expect(block, contains('6 further cause(s) omitted'));
     expect(block.length, lessThanOrEqualTo(500));
   });
+
+  test('equal scores are ordered by file, line and rule, not by chance', () {
+    // Sorting by score alone left ties in an order List.sort does not promise
+    // to keep, so two runs could hand an agent a different first action.
+    Finding dead(String file, int line, String event) => _finding(
+      rule: ruleDeadHandler,
+      severity: PipeSeverity.info,
+      file: file,
+      line: line,
+      data: {'event': event},
+    );
+    final plan = buildWorkPlan(
+      root: '/root',
+      report: _report([
+        dead('lib/z.dart', 1, 'Z'),
+        dead('lib/a.dart', 9, 'A9'),
+        dead('lib/a.dart', 2, 'A2'),
+        dead('lib/m.dart', 5, 'M'),
+      ]),
+    );
+    expect(plan.ordered.map((action) => action.label), ['A2', 'A9', 'M', 'Z']);
+  });
 }