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/engines/regex/detect-text.mjs

@@ -1,4 +1,4 @@
-import { GENERIC_FONTS, OVERUSED_FONTS } from '../../shared/constants.mjs';
+import { GENERIC_FONTS, OVERUSED_FONTS, EM_DASH_FLOOR, EM_DASH_CHARS_PER_DASH } from '../../shared/constants.mjs';
 import { isNeutralColor } from '../../shared/color.mjs';
 import { extractGoogleFontFamilies } from '../../shared/fonts.mjs';
 import { checkSourceDesignSystem } from '../../design-system.mjs';
@@ -16,6 +16,7 @@ const hasRounded = (line) => /\brounded(?:-\w+)?\b/.test(line);
 const hasBorderRadius = (line) => /border-radius/i.test(line);
 const isSafeElement = (line) => /<(?:blockquote|nav[\s>]|pre[\s>]|code[\s>]|a\s|input[\s>]|span[\s>])/i.test(line);
 
+
 /** Strip HTML to plain text — drops script/style/comments/tags so
  *  content-text analyzers don't false-positive on code or CSS. */
 function stripHtmlToText(html) {
@@ -306,9 +307,16 @@ const REGEX_ANALYZERS = [
     const dominant = Object.entries(counts).sort((a, b) => b[1] - a[1])[0][0];
     return [finding('monotonous-spacing', filePath, `~${dominant}px used ${maxCount}/${rounded.length} times (${Math.round(pct * 100)}%)`)];
   },
-  // Em-dash overuse: 5+ em-dashes or "--" in body text content
-  // (occasional em-dash use in prose is fine; the pattern fires only
-  // when count crosses into AI-cadence territory).
+  // Em-dash overuse (ADVISORY): the AI cadence tell is em-dash *saturation*,
+  // not the occasional dash. Humans use em-dashes legitimately, so this rule is
+  // advisory (surfaced separately, never a failure, hook-skipped by default) and
+  // its threshold is deliberately conservative. Two gates must both hold:
+  //   1. Absolute floor of EM_DASH_FLOOR (8) dashes — a page with a handful
+  //      never fires, no matter how short.
+  //   2. Density: at least one dash per EM_DASH_CHARS_PER_DASH (500) characters
+  //      of body text, so a long article that uses eight across several thousand
+  //      words is left alone while a short, dash-per-clause landing page is not.
+  // Raised from the old flat 5-dash floor, which fired on ordinary long prose.
   //
   // stripHtmlToText drops tags but leaves character-entity escapes intact, so
   // a model that writes `&mdash;`, `&#8212;`, or `&#x2014;` renders an em-dash
@@ -322,7 +330,11 @@ const REGEX_ANALYZERS = [
     let count = 0;
     const re = /[—]|--(?=\S)/g;
     while (re.exec(text) !== null) count++;
-    if (count < 5) return [];
+    if (count < EM_DASH_FLOOR) return [];
+    // Saturation gate: dashes must be dense in the prose, not sprinkled through
+    // a long document. textLength <= count * chars-per-dash means the density is
+    // at or above the threshold.
+    if (text.length > count * EM_DASH_CHARS_PER_DASH) return [];
     return [finding('em-dash-overuse', filePath, `${count} em-dashes in body text`)];
   },
   // Marketing buzzwords: SaaS phrase list