fix(diff): surface same-severity in-place finding changes as [CHANGED]

test/heuristics.test.mjs

@@ -1,3 +1,3 @@
 import test from 'node:test';
 import assert from 'node:assert/strict';
 import { fileURLToPath } from 'node:url';
@@ -340,6 +340,41 @@
   assert.equal(delta.resolvedFindings, undefined);
 });
 
+test('diffReports: same kind/subject/file and severity but different message is surfaced as [CHANGED]', () => {
+  // A medium finding mutated into a *different* medium at the same
+  // place. The (kind, subject, file) key matches, so without a content
+  // check this would be dropped as pre-existing.
+  const base = makeReport([makeFinding({ message: 'github launches npx server-github.' })]);
+  const head = makeReport([makeFinding({ message: 'github launches npx server-evil.' })]);
+
+  const delta = diffReports(base, head);
+  assert.equal(delta.findingCount, 1);
+  assert.equal(delta.findings[0].severity, 'medium');
+  assert.match(delta.findings[0].message, /^\[CHANGED\]/);
+  assert.match(delta.findings[0].message, /server-evil/);
+});
+
+test('diffReports: same severity with a changed signature is surfaced as [CHANGED]', () => {
+  // When reports carry signatures (the audit path always signs), the
+  // content identity comes from the signature rather than the message.
+  const base = makeReport([makeFinding({ signature: 'aaaaaaaaaaaaaaaa' })]);
+  const head = makeReport([makeFinding({ signature: 'bbbbbbbbbbbbbbbb' })]);
+
+  const delta = diffReports(base, head);
+  assert.equal(delta.findingCount, 1);
+  assert.match(delta.findings[0].message, /^\[CHANGED\]/);
+});
+
+test('diffReports: same severity and identical signature stays out of the delta', () => {
+  // Identical signatures mean the violation is unchanged even if some
+  // unrelated report field differs — it is pre-existing, not a regression.
+  const base = makeReport([makeFinding({ signature: 'cafef00dcafef00d', message: 'phrasing A' })]);
+  const head = makeReport([makeFinding({ signature: 'cafef00dcafef00d', message: 'phrasing B' })]);
+
+  const delta = diffReports(base, head);
+  assert.equal(delta.findingCount, 0);
+});
+
 test('makeMeshContext: indexes servers by name across surfaces in a single pass', () => {
   const policies = {
     mcpSurfaces: [