feat(seo-data): C3 — internal link graph; orphans + click depth, measured

seo-analyzer.md:613 asks "Every important page reachable within 3 clicks?"
and :616 asks "Orphan pages (no inbound internal links)?". Neither ever had a
command — same shape as the sameAs check before W3. This is that command.

My earlier reservation ("costs a lot of network") was wrong and the
measurement killed it: 24 pages in 2.7s, 86 in 3.8s. Cheap enough to always
run on FULL.

EXHAUSTIVE OR NOTHING is the design constraint, not a nicety. Orphans cannot
be sampled: proving a page has no inbound link means having read every other
page. So when the crawl is capped or any page fails, orphans are WITHHELD —
`orphans_withheld: true` and no list. A false orphan ("page X has no inbound
links" when it does) sends a client fixing what is not broken; that is the
worst finding this tool could emit. The cap does not degrade the result, it
invalidates it.

SPA refusal: on a client-rendered site the links are not in the HTML and
every page reads as orphaned. That is catastrophic, so an empty graph returns
degraded/no_links_in_html instead of a full false-positive list. No JS
rendering by design — that is the R1/R2 arbitration, not something to smuggle
in here.

Verified against BOTH live sites and against a planted failure, because two
clean results are not evidence a detector detects:
- native PHP: 24 pages, 335 links, depth 2, 0 orphans
- Astro: 86 pages, 2015 links, depth 2, 0 orphans
- fixture with a planted orphan + a 4-click chain: both found. Filters proven
  on real shapes seen live — /css/main.css?v=1778157313, #anchors, mailto:,
  tel:, external hosts, .png. /b/ in markup vs /b in sitemap unify to one node
  rather than a phantom orphan pair.

Fixed a flaw in my own mock while writing that test: a single page.html
fixture cannot express a GRAPH (every node gets identical links), so the mock
is now pages.json = {url: html}.

Verified: seo-data 122 -> 136 pass, 0 fail; full suite green; shellcheck +
py_compile clean.

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

@@ -135,6 +135,33 @@ has "billion-laughs refused"     "$DTD" '"status": "degraded"'
 has "dtd reason is distinct"     "$DTD" 'unsafe_xml_dtd'
 hasnt "dtd never parsed"         "$DTD" '"count"'
 
+echo "── linkgraph ──"
+LG="$(SEO_DATA_MOCK_DIR="$SD/fixtures-linkgraph" python3 "$SD/linkgraph.py" \
+  --url https://ex.com/sitemap.xml)"
+has "linkgraph ok"               "$LG" '"status": "ok"'
+has "linkgraph crawls all"       "$LG" '"pages_crawled": 7'
+# THE test: a planted page nobody links to must be found. Two live sites both
+# returned zero orphans; without this, "always returns []" looks identical.
+has "finds the planted orphan"   "$LG" '"https://ex.com/orphan"'
+has "orphan is also unreachable" "$LG" '"unreachable"'
+has "depth chain measured"       "$LG" '"max_depth": 4'
+has "flags >3 clicks"            "$LG" '"https://ex.com/deepest"'
+# home links: /a and /b only. anchor, .css?v=, mailto:, tel:, external, .png
+# are not page links — 9 total across the 7 pages.
+has "filters non-page links"     "$LG" '"total_internal_links": 9'
+hasnt "no external host"         "$LG" 'other.com'
+hasnt "no asset link"            "$LG" 'main.css'
+hasnt "no image link"            "$LG" 'logo.png'
+# /b/ in the markup vs /b in the sitemap must be ONE node, not a phantom orphan
+hasnt "trailing slash unified"   "$LG" '"https://ex.com/b/"'
+
+# An orphan from a partial crawl is a false orphan: withhold, do not truncate.
+CAP="$(SEO_DATA_MOCK_DIR="$SD/fixtures-linkgraph" python3 "$SD/linkgraph.py" \
+  --url https://ex.com/sitemap.xml --max 3)"
+has "cap is reported"            "$CAP" '"capped": true'
+has "capped withholds orphans"   "$CAP" '"orphans_withheld": true'
+hasnt "capped emits no orphans"  "$CAP" '"orphans":'
+
 echo "── fetch.sh ──"
 FETCH="$SD/fetch.sh"
 # SEO_DATA_ENV_FILE=/dev/null: tests must NEVER source the real ~/.claude/.env —