bake: the anchor must have matched one element on the page

The lasting rules a bake appends apply to every element the anchor
matches, so a `tag.class` anchor shared by siblings (three cards from one
JSX element, say) restyled all of them, not the element the user picked.
The overlay now journals, with the generate event, the anchor it would
bake on (`element.anchor`: the id, else the tag with its classes) and how
many elements matched it when Go fired (`element.anchorMatches`). The
planner bakes only when the source anchor is that same selector and the
count is one; otherwise it leaves the carbonize block with the count in
`bakeSkipped`, and the agent integrates the variant by hand.

Written with AI assistance (Claude).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tests/live-browser-source.test.mjs

@@ -17,6 +17,13 @@ describe('live-browser source contracts', () => {
     }
   });
 
+  it('describes the picked element with the anchor a bake would use and its match count', () => {
+    const body = SOURCE.match(/function extractContext\(el\) \{[\s\S]*?\n  \}/)?.[0];
+    assert.ok(body);
+    assert.match(body, /anchorMatches = document\.querySelectorAll\(anchor\)\.length/);
+    assert.match(body, /anchor, anchorMatches,/);
+  });
+
   for (const annotated of [false, true]) {
     for (const outcome of ['created', 'failed', 'superseded']) {
       it(`${annotated ? 'annotated' : 'plain'} generation checkpoints only its acknowledged current session (${outcome})`, async () => {