docs(limitations): the raw-html FP rate was blamed on the wrong bullet

The "Measured, document by document" bullet attributed `active:raw-html` in
52 of vendor-harvest's 98 to the relative-URL-attribute over-reach. Measured
by re-running the gate with that branch narrowed to external targets only, it
frees 1 document there, 1 in generated-notes, 3 in reference-corpus. The
over-reach is real and nearly free; the rate is driven by the NAME branch
(`<a>` 298, `<frame>` 94, `<img>` 63 per wiki corpus; `<base>` 113 in
reference-corpus).

Two further corrections the census forced:

- The over-reach is not only MDX. In reference-corpus it lands on Azure APIM
  policy XML (`<set-header>` 50, `<ip-filter>` 3, `<set-query-parameter>` 2).
- New bullet: APIM's attribute-less `<base />` collides with HTML's `<base>`,
  whose whole affordance is `href`. 30 of reference-corpus's 389 documents.

Also pins corpus provenance. "Every population was swept twice and reproduced
its counts exactly" is false for two of the three: vendor-harvest and
generated-notes live in claude-code-llm-wiki, which re-harvests per Claude
Code release. At its commit aba87e2 they now give 100/187 and 90/550;
reference-corpus is static and reproduced 133/389 exactly. The published table
keeps its original counts — measurement provenance is never silently bumped —
and gains the provenance it was missing.

README limitations count 33 -> 34.

README.md

@@ -242,7 +242,7 @@ a green scan means safe content. The highest-impact items:
   exact constructs the gate grades. Budget human review, or run a source you actually
   trust as trusted. Re-run it yourself with [`docs/fp-sweep.py`](docs/fp-sweep.py).
 
-**Full list — 33 items, each with the mechanism, plus the out-of-scope boundary:**
+**Full list — 34 items, each with the mechanism, plus the out-of-scope boundary:**
 [`docs/LIMITATIONS.md`](docs/LIMITATIONS.md). Several carry field measurements from
 consumer corpora, including the false positives the URL-shape rule actually produces.