Make em-dash-overuse an advisory rule with browser parity

Em-dashes are used legitimately by humans, so em-dash-overuse fired far too
often. Reclassify it as the first advisory-tier rule: detected, but never a
failure.

Engine
- Add `advisory: true` to the rule metadata schema (em-dash-overuse is the
  first). findings.mjs stamps `advisory: true` on advisory findings so every
  consumer can partition without a registry lookup. Rule count stays 58.
- Raise the firing threshold from a flat 5 dashes to two gates: an absolute
  floor of 8 and a density of about one dash per 500 characters of body text.
  A long article that uses a few em-dashes no longer trips; a short,
  dash-per-clause page still does. Entity decoding (mdash, numeric, hex) is
  unchanged. Thresholds live in shared/constants.mjs so every engine agrees.

Browser parity
- The browser bundle carried a registry entry but no logic, so the overlay and
  extension could never flag it. Add checkEmDashOveruse / checkEmDashOveruseDOM
  in rules/checks.mjs (reads rendered text, no entity decoding needed), wire it
  into the injected page-level pass, and carry the advisory flag through
  serializeFindings so the overlay/extension can render it with the mildest
  affordance.

CLI
- Advisory findings print under a separate dimmed "Advisory" section, are
  excluded from the failure count, and never change the exit code (an
  advisory-only scan exits 0). JSON keeps them with `"advisory": true`.
  `--no-advisory` suppresses them entirely.

Hook
- Advisory rules are skipped by default in both the per-edit and Stop deep-pass
  hooks, so the hook never nags about them. Opt in with
  `.impeccable/config.json` -> `detector.advisoryRules: "include"`.

Tests
- Fixture + threshold + browser-adapter coverage; advisory-skip default and
  opt-in for the hook; formatFindings partitioning. The em-dash-overuse stand
  for a deferred copy rule in the tier tests is swapped to marketing-buzzword.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

cli/engine/rules/checks.mjs

@@ -1,5 +1,7 @@
 import {
   BORDER_SAFE_TAGS,
+  EM_DASH_CHARS_PER_DASH,
+  EM_DASH_FLOOR,
   GENERIC_FONTS,
   KNOWN_SERIF_FONTS,
   OVERUSED_FONTS,
@@ -2594,6 +2596,33 @@ function checkNumberedSectionLabelsDOM() {
   return checkNumberedSectionLabels({ candidates });
 }
 
+// Em-dash overuse (ADVISORY) — pure logic shared by the browser DOM check.
+// Mirrors the regex/static-HTML analyzer in engines/regex/detect-text.mjs:
+// two gates (absolute floor + density) so a long article using a few dashes is
+// left alone while a short, dash-per-clause page is flagged. Operates on
+// already-rendered text, so no HTML-entity decoding is needed (the browser has
+// resolved `—` to the literal glyph). Exported for jsdom unit tests.
+function checkEmDashOveruse(text) {
+  const body = typeof text === 'string' ? text.replace(/\s+/g, ' ') : '';
+  let count = 0;
+  const re = /[—]|--(?=\S)/g;
+  while (re.exec(body) !== null) count++;
+  if (count < EM_DASH_FLOOR) return [];
+  if (body.length > count * EM_DASH_CHARS_PER_DASH) return [];
+  return [{ id: 'em-dash-overuse', snippet: `${count} em-dashes in body text` }];
+}
+
+function checkEmDashOveruseDOM() {
+  const body = document.body;
+  if (!body) return [];
+  // innerText reflects rendered, visible text; fall back to textContent for
+  // engines (jsdom) that don't compute innerText.
+  const text = typeof body.innerText === 'string' && body.innerText
+    ? body.innerText
+    : (body.textContent || '');
+  return checkEmDashOveruse(text);
+}
+
 function checkElementMotionDOM(el) {
   const tag = el.tagName.toLowerCase();
   if (SAFE_TAGS.has(tag)) return [];
@@ -5121,6 +5150,8 @@ export {
   checkNumberedSectionLabels,
   checkNumberedSectionLabelsFromDoc,
   checkNumberedSectionLabelsDOM,
+  checkEmDashOveruse,
+  checkEmDashOveruseDOM,
   isRepeatedTextContainer,
   collectRepeatedContainerTextFindings,
   checkRepeatedContainerTextFromDoc,