feat(seo-data): C1b — sitemap verb, the denominator COVERAGE never had

I5 made a COVERAGE line mandatory in STEP 9 and told the agent to "count the
URLs in sitemap.xml" without giving it a command. STEP 4 only ever did
`curl … | head -50` — a preview, not a count. This closes that.

fetch.sh sitemap --url … → {count, urls[], index, dropped}. Stdlib only
(urllib + xml.etree + gzip): no auth, no Google, no venv, so it runs wherever
the mock/degrade paths run. Follows <sitemapindex> one level, dedupes, strips
whitespace, handles .xml.gz. Every cap REPORTS what it cut (children_skipped,
truncated) rather than truncating silently — same rule as COVERAGE itself.

PLAN CORRECTION: the proposal said the verb would "validate each URL via the
H1 guard". Wrong. urllib fetches these, so nothing here reaches a shell and
there is no injection surface to guard. The guard belongs at the point of
use, where seo-analyzer interpolates a URL into curl — which is the contract
the sameAs check already established. A second copy of url-guard here would
only drift from the first. The module carries a garbage filter, named as such.

SECURITY: the security-guidance hook asked for defusedxml. Taken seriously,
not obeyed — it would drag a venv into a module whose whole point is being
stdlib-only. Split the threat instead: xml.etree does NOT expand external
entities (XXE is not the vector), but it IS billion-laughs-vulnerable, and
the 20 MB read ceiling bounds the input, not the expansion. A sitemap NEVER
has a DTD — sitemaps.org is <?xml?> then <urlset xmlns=> — so any
doctype/entity is refused BEFORE parsing, with its own reason
(unsafe_xml_dtd, distinct from parse_failed: it is a finding, not a glitch).
Refusing the construct beats depending on parser internals. Fixture is a real
billion-laughs payload.

Verified against the live target, not just fixtures: zenquality's sitemap
returns count=86, dropped=0, matching `grep -c '<loc>'` on the raw XML
exactly. Dead URL → {"status":"degraded","reason":"fetch_failed"}, exit 0.
seo-data 95 -> 110 pass, 0 fail; full suite green; shellcheck + py_compile
clean.

Note: no config-edit sentinel was needed after all — config-protection guards
lib/tests, not lib/seo-data. I posted one, found it uncommitted-and-unconsumed
afterwards, and removed it rather than leave an open one-shot gate lying
around. Worth knowing: seo-data.test.sh is 110 assertions and is NOT covered
by that hook, while lib/tests/*.test.sh is.

lib/seo-data/seo-data.test.sh

@@ -81,6 +81,36 @@ has "gsc degrades w/o creds"     "$DEG" '"status": "degraded"'
 has "gsc degrade reason"         "$DEG" 'no_credentials'
 rm -rf "$TMP2"
 
+echo "── sitemap ──"
+SM="$(SEO_DATA_MOCK_DIR="$MOCK" python3 "$SD/sitemap.py" --url https://ex.com/sitemap.xml)"
+has "sitemap ok"                 "$SM" '"status": "ok"'
+has "sitemap not an index"       "$SM" '"index": false'
+# fixture holds 8 <loc>: 1 empty, blog twice, ftp:// and a quoted one to drop
+has "sitemap dedupes"            "$SM" '"count": 4'
+has "sitemap counts drops"       "$SM" '"dropped": 2'
+has "sitemap strips whitespace"  "$SM" '"https://ex.com/spaced"'
+hasnt "sitemap drops non-http"   "$SM" 'ftp://'
+hasnt "sitemap drops shell-meta" "$SM" 'bad"quote'
+# namespace-agnostic: real sitemaps carry sitemaps.org xmlns (+ xhtml here)
+has "sitemap reads namespaced"   "$SM" '"https://ex.com/services"'
+
+IDX="$(SEO_DATA_MOCK_DIR="$SD/fixtures-sitemap-index" python3 "$SD/sitemap.py" \
+  --url https://ex.com/sitemap.xml)"
+has "sitemapindex detected"      "$IDX" '"index": true'
+has "sitemapindex fans out"      "$IDX" '"children_read": 2'
+has "sitemapindex no child fail" "$IDX" '"children_failed": 0'
+has "sitemapindex yields urls"   "$IDX" '"https://ex.com/child-a"'
+
+# A sitemap NEVER has a DTD. Refused at the door: xml.etree does not expand
+# external entities but IS billion-laughs-vulnerable, and the 20MB read ceiling
+# bounds the input, not the expansion. Refusing beats depending on the parser,
+# and keeps this module stdlib-only (no defusedxml, no venv).
+DTD="$(SEO_DATA_MOCK_DIR="$SD/fixtures-sitemap-dtd" python3 "$SD/sitemap.py" \
+  --url https://ex.com/sitemap.xml)"
+has "billion-laughs refused"     "$DTD" '"status": "degraded"'
+has "dtd reason is distinct"     "$DTD" 'unsafe_xml_dtd'
+hasnt "dtd never parsed"         "$DTD" '"count"'
+
 echo "── fetch.sh ──"
 FETCH="$SD/fetch.sh"
 # SEO_DATA_ENV_FILE=/dev/null: tests must NEVER source the real ~/.claude/.env —