fix: bound CSS seeding to real matches and ownership before supersession removal

Addresses two cursor findings on extractMatchingSourceCss plus an adjacent
hazard in the same removal machinery:

- Class matching is token-bounded, never substring: .btn no longer seeds
  .btn-primary and .stage no longer seeds .stages. A falsely seeded
  selector was an accept-time deletion of a hand-written rule, since any
  seeded selector the variant does not re-declare is removed as
  superseded.
- Tag rules that style the pick (h1, a, p) now seed the preview stub, so
  unclassed selections start from the real cascade. They are excluded
  from the supersedable set: tag rules style shared elements across the
  route and must never be removal candidates.
- Supersession removal is now bounded by ownership: a seeded class
  selector whose class is still used by markup OUTSIDE the replaced
  region survives the accept, because removing it would strip styling
  from markup the accept never touched.

Tests cover substring non-matches, tag seeding with a tag-free
supersedable set, and a shared-class accept where .card is used both
inside the pick and elsewhere.

AI-assisted (Claude Code).

Co-Authored-By: Claude Code <noreply@anthropic.com>

tests/live-svelte-component-accept.test.mjs

@@ -226,11 +226,43 @@ describe('svelte component scaffold + accept pipeline', () => {
   });
 
   it('extractMatchingSourceCss picks only rules that style the selection', () => {
-    const css = extractMatchingSourceCss(ROUTE_SOURCE, '<ol class="pit-board"><li class="stage">x</li></ol>');
+    const { css } = extractMatchingSourceCss(ROUTE_SOURCE, '<ol class="pit-board"><li class="stage">x</li></ol>');
     assert.match(css, /\.pit-board/);
     assert.match(css, /\.stage/);
     assert.doesNotMatch(css, /\.footer/);
   });
+
+  it('class matching is token-bounded, never substring', () => {
+    // The field hazard: a falsely seeded selector becomes an accept-time
+    // DELETION of a hand-written rule the pick never used.
+    const route = `<style>
+  .btn { color: red; }
+  .btn-primary { color: blue; }
+  .stage { padding: 4px; }
+  .stages { display: grid; }
+</style>`;
+    const { css, supersedable } = extractMatchingSourceCss(route, '<button class="btn"><span class="stage">x</span></button>');
+    assert.match(css, /\.btn \{/);
+    assert.match(css, /\.stage \{/);
+    assert.doesNotMatch(css, /btn-primary/, '.btn must not seed .btn-primary');
+    assert.doesNotMatch(css, /\.stages/, '.stage must not seed .stages');
+    assert.deepEqual([...supersedable].sort(), ['.btn', '.stage']);
+  });
+
+  it('tag rules seed the preview but are never supersedable', () => {
+    const route = `<style>
+  h1 { font-size: 3rem; }
+  h1.hero { letter-spacing: -0.02em; }
+  p { line-height: 1.6; }
+  .sidebar { width: 20rem; }
+</style>`;
+    const { css, supersedable } = extractMatchingSourceCss(route, '<h1 class="hero">Title</h1>');
+    assert.match(css, /h1 \{ font-size/, 'bare tag rules that style the pick are seeded');
+    assert.match(css, /h1\.hero/, 'class rules still seed');
+    assert.doesNotMatch(css, /^p \{/m, 'unrelated tags are not seeded');
+    assert.doesNotMatch(css, /\.sidebar/);
+    assert.deepEqual([...supersedable], ['h1.hero'], 'only class-matched selectors may be removed on accept');
+  });
 });
 
 describe('review regressions: preview-truth supersession (the Pitch mangle)', () => {
@@ -421,3 +453,88 @@ describe('review regressions: publish-time compile gate', () => {
     }
   });
 });
+
+describe('review regressions: shared-class supersession guard', () => {
+  const SHARED_SOURCE = `<script>
+  let items = [{ name: 'a' }, { name: 'b' }];
+</script>
+
+<section class="page">
+  <div class="card intro">Intro copy stays here.</div>
+  <ul class="list">
+    {#each items as item}
+      <li class="card">{item.name}</li>
+    {/each}
+  </ul>
+</section>
+
+<style>
+  .page { padding: 24px; }
+  .card { border: 1px solid #999; border-radius: 8px; }
+  .list { display: grid; gap: 8px; }
+</style>
+`;
+
+  it('keeps a superseded selector whose class is still used outside the replaced region', () => {
+    const tmp4 = realpathSync(mkdtempSync(join(tmpdir(), 'impeccable-shared-class-')));
+    try {
+      mkdirSync(join(tmp4, 'node_modules'), { recursive: true });
+      try {
+        symlinkSync(join(REPO_NODE_MODULES, 'svelte'), join(tmp4, 'node_modules', 'svelte'), 'dir');
+      } catch {
+        cpSync(join(REPO_NODE_MODULES, 'svelte'), join(tmp4, 'node_modules', 'svelte'), { recursive: true });
+      }
+      write(tmp4, 'package.json', JSON.stringify({ name: 'app' }));
+      write(tmp4, 'src/lib/Shared.svelte', SHARED_SOURCE);
+
+      // Pick the <ul class="list"> block. Its items use .card, and so does
+      // the intro div OUTSIDE the pick.
+      const lines = SHARED_SOURCE.split('\n');
+      const startLine = lines.findIndex((l) => l.includes('class="list"')) + 1;
+      const endLine = lines.findIndex((l) => l.trim() === '</ul>') + 1;
+      const originalLines = lines.slice(startLine - 1, endLine);
+
+      const session = scaffoldSvelteComponentSession({
+        id: 'shared01',
+        count: 1,
+        sourceFile: 'src/lib/Shared.svelte',
+        sourceStartLine: startLine,
+        sourceEndLine: endLine,
+        originalLines,
+        cwd: tmp4,
+      });
+      assert.equal(session.fallback, undefined, session.reason);
+      assert.equal(session.manifest.seededSelectors.includes('.card'), true, 'the pick uses .card, so it seeds');
+
+      // The variant re-declares .list but NOT .card.
+      write(tmp4, join(session.componentDir, 'v1.svelte'), `<script>
+  let { items = [] } = $props();
+</script>
+
+<ul class="list">
+  {#each items as item}
+    <li class="card">{item.name}</li>
+  {/each}
+</ul>
+
+<style>
+  .list { display: flex; flex-direction: column; gap: 12px; }
+</style>
+`);
+      const manifest = findSvelteComponentManifest('shared01', tmp4);
+      const result = inlineSvelteComponentAccept(manifest, 1, null, tmp4);
+      assert.equal(result.handled, true, result.error);
+      const out = readFileSync(join(tmp4, 'src/lib/Shared.svelte'), 'utf-8');
+
+      // .card is shared with the intro div outside the replaced region:
+      // removing it would strip styling from markup this accept never
+      // touched, so it must survive despite not being re-declared.
+      assert.match(out, /\.card \{ border: 1px solid #999; border-radius: 8px; \}/);
+      assert.equal(result.css.superseded.includes('.card'), false);
+      // The re-declared .list took the variant's shape.
+      assert.match(out, /\.list \{ display: flex/);
+    } finally {
+      rmSync(tmp4, { recursive: true, force: true });
+    }
+  });
+});