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/shared/constants.mjs

@@ -68,6 +68,15 @@ const GENERIC_FONTS = new Set([
 const WCAG_LARGE_TEXT_PX = 18 * (96 / 72);
 const WCAG_LARGE_BOLD_TEXT_PX = 14 * (96 / 72);
 
+// Em-dash overuse (advisory) thresholds, shared by the regex/static-HTML
+// analyzer and the browser DOM check so both fire on the same saturation
+// pattern. Two gates must hold: an absolute floor of EM_DASH_FLOOR dashes, and
+// a density of at least one dash per EM_DASH_CHARS_PER_DASH characters of body
+// text. A long article that uses a few em-dashes is left alone; a short,
+// dash-per-clause page is not.
+const EM_DASH_FLOOR = 8;
+const EM_DASH_CHARS_PER_DASH = 500;
+
 // Serif faces that show up in italic-display heroes. The rule also fires when
 // the primary face is unknown but the stack ends in the generic `serif` token,
 // which catches custom/private faces with a serif fallback.
@@ -97,5 +106,7 @@ export {
   GENERIC_FONTS,
   WCAG_LARGE_TEXT_PX,
   WCAG_LARGE_BOLD_TEXT_PX,
+  EM_DASH_FLOOR,
+  EM_DASH_CHARS_PER_DASH,
   KNOWN_SERIF_FONTS,
 };