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/fetch.sh

@@ -32,6 +32,8 @@ case "$cmd" in
   # No auth, no Google: stdlib-only, runs even without the venv.
   sitemap)
     exec "$PY" "$HERE/sitemap.py" --store "$STORE" "$@" ;;
+  drift)
+    exec "$PY" "$HERE/drift.py" --store "$STORE" "$@" ;;
   rendercheck)
     exec "$PY" "$HERE/render_check.py" --store "$STORE" "$@" ;;
   linkgraph)
@@ -48,6 +50,6 @@ case "$cmd" in
     fi
     echo '{"status":"error","reason":"usage: fetch.sh forget {--label <label>|--all} (label charset: A-Za-z0-9._-)"}'
     exit 2 ;;
-  *) echo '{"status":"error","reason":"usage: fetch.sh {accounts|crux|queries|inspect|cannibal|sitemap|rendercheck|linkgraph|forget} [flags]"}'
+  *) echo '{"status":"error","reason":"usage: fetch.sh {accounts|crux|queries|inspect|cannibal|sitemap|rendercheck|linkgraph|drift|forget} [flags]"}'
      exit 2 ;;
 esac