feat(seo-data): H2 — drift baseline; regressions vs changes, not prose

seo-analyzer.md:1365 keeps history as "date + score + key changes" — prose the
LLM writes about its own previous prose. Lossy, unreproducible, and
machine-uncomparable, so "the redesign silently dropped 40 canonicals" is
invisible unless someone happens to notice.

drift snapshots title/description/canonical/robots/h1_count/jsonld_types per
URL and diffs them. Stdlib only, no auth.

The classification IS the feature: LOSING a signal is a regression, CHANGING
one is a change that may well be intended. The engine says which kind; the
agent judges. A reworded title is not an alert; an evaporated canonical is.

Runs over the WHOLE sitemap, never a sample — caught while designing: a drift
computed over a sample that changes between runs compares nothing.

NOT rank tracking. That is the common misread of this same feature elsewhere;
positions come from GSC `queries`. This is on-page regression detection.

Also caught in my own draft before testing: _capture reused
sm._mock("page.html"), the exact single-fixture flaw I had already fixed in
linkgraph — one fixture cannot express a multi-page snapshot, every URL would
read identical. Now pages.json, same convention.

Proved on a planted failure rather than a happy path — two clean sites would
look identical to a detector that always returns []:
  v1 -> v2: canonical lost on /a, h1 + jsonld lost on /, title reworded,
  /gone removed, /neuve added
  → 3 regressions, 1 change, gone/new both detected, title correctly NOT a
    regression.

Store is ~/.claude/seo-data/drift/<host>.json, 0700, written via os.replace so
a crash never leaves a half-written baseline; a corrupt store degrades to
"first run" instead of killing the audit.

Verified: seo-data 144 -> 155 pass, 0 fail; full suite green.

lib/seo-data/fixtures-drift-v1/pages.json

@@ -0,0 +1,5 @@
+{
+  "https://ex.com/": "<html><head><title>Accueil</title><meta name='description' content='desc'><link rel='canonical' href='https://ex.com/'><script type='application/ld+json'>{\"@type\":\"LocalBusiness\"}</script></head><body><h1>Accueil</h1></body></html>",
+  "https://ex.com/a": "<html><head><title>Page A</title><link rel='canonical' href='https://ex.com/a'></head><body><h1>A</h1></body></html>",
+  "https://ex.com/gone": "<html><head><title>Bientot supprimee</title></head><body><h1>G</h1></body></html>"
+}