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/seo-data.test.sh

@@ -178,6 +178,32 @@ has "cap is reported"            "$CAP" '"capped": true'
 has "capped withholds orphans"   "$CAP" '"orphans_withheld": true'
 hasnt "capped emits no orphans"  "$CAP" '"orphans":'
 
+echo "── drift (H2) ──"
+DH="$(mktemp -d)"
+D1="$(HOME="$DH" SEO_DATA_MOCK_DIR="$SD/fixtures-drift-v1" python3 "$SD/drift.py" \
+  --url https://ex.com/sitemap.xml)"
+has "first run is a baseline"    "$D1" '"baseline": true'
+has "baseline captures pages"    "$D1" '"pages": 3'
+hasnt "baseline diffs nothing"   "$D1" '"regressions"'
+# v2: canonical lost on /a, h1+jsonld lost on /, title reworded, /gone removed,
+# /neuve added. Losses are regressions; a reworded title is not.
+D2="$(HOME="$DH" SEO_DATA_MOCK_DIR="$SD/fixtures-drift-v2" python3 "$SD/drift.py" \
+  --url https://ex.com/sitemap.xml)"
+has "second run diffs"           "$D2" '"baseline": false'
+has "detects removed url"        "$D2" '"https://ex.com/gone"'
+has "detects added url"          "$D2" '"https://ex.com/neuve"'
+has "lost canonical = regression" "$D2" '"canonical"'
+has "lost h1 = regression"       "$D2" '"h1_count"'
+has "lost jsonld = regression"   "$D2" '"jsonld_types"'
+# the classification IS the feature: losing a signal != changing one
+NREG="$(printf '%s' "$D2" | python3 -c 'import sys,json; print(len(json.load(sys.stdin)["regressions"]))')"
+NCHG="$(printf '%s' "$D2" | python3 -c 'import sys,json; print(len(json.load(sys.stdin)["changes"]))')"
+[ "$NREG" = "3" ] && ok "3 losses classed as regressions" \
+                  || no "3 losses classed as regressions" "got $NREG"
+[ "$NCHG" = "1" ] && ok "reworded title is a change, not a regression" \
+                  || no "reworded title is a change, not a regression" "got $NCHG"
+rm -rf "$DH"
+
 echo "── fetch.sh ──"
 FETCH="$SD/fetch.sh"
 # SEO_DATA_ENV_FILE=/dev/null: tests must NEVER source the real ~/.claude/.env —