docs(census): the two wiki corpora are measured, and the tightening is thinner than zero looked

0.7.0 shipped with its corpus numbers deliberately absent: the census had only
run on reference-corpus, which the 0.6.0 narrowing had already emptied of
raw-HTML drivers, so it bounded the change rather than showing it. Both wiki
corpora are now measured through the pinned instrument, in one session because
they are living populations.

Under PRESET_USER_UPLOAD, 0.6.0 as shipped -> 0.7.0, each population against its
own denominator (the two wiki corpora share content and are never summed):

  reference-corpus   389 docs   54 -> 53   ceiling 53
  vendor-harvest     187 docs   62 -> 20   ceiling 18   42 of 44 achievable
  generated-notes    552 docs   59 -> 15   ceiling 13   44 of 46 achievable

PRODUCTION matched C1 + D field for field in every population, which is the
check that the instrument and the shipped predicate have not drifted.

Two things the measurement changed rather than confirmed:

The pair is superadditive by 13 documents in BOTH wiki corpora. Alone, the split
frees 8 in each and the narrowing 21 and 23; together they free 42 and 44.
Shipping either alone would have measured as barely worth the label.

The TIGHTENS column reads 0 on both tiers in all three populations, but that zero
is empirical and thin: the split measured ALONE tightens 13 documents on the
trusted tier in vendor-harvest and 14 in generated-notes, and the narrowing
cancels each one. LIMITATIONS now says so explicitly, so nobody reads the zero as
'cannot happen' -- the escalation is still constructed and pinned by
test_split_tightens_the_trusted_tier_when_both_carriers_are_present.

generated-notes counted 552 documents, not the 550 the scratchpad probe saw.
Living corpus, measured fresh.

791 passed; coverage 129/129, 6/6 documented gaps hold.

docs/LIMITATIONS.md

@@ -275,9 +275,20 @@ items; this is the full list, each with the mechanism.
   `tests/test_wiring.py::test_split_tightens_the_trusted_tier_when_both_carriers_are_present`,
   and `docs/rawhtml-census.py` now reports a `TIGHTENS` column against the
   previously-shipped row on both trust tiers — "frees N" without "tightens M" is a
-  one-sided number. **Measured so far: 0 documents tightened on reference-corpus
-  (389). The two wiki corpora are NOT yet re-measured; until they are, the size of
-  this cost is unknown, not zero.**
+  one-sided number. **Measured on all three populations, as shipped: 0 documents
+  tightened, on both trust tiers — reference-corpus (389), vendor-harvest (187),
+  generated-notes (552).**
+  **That zero is empirical, not structural, and the census shows exactly how thin
+  it is.** The split measured *alone* tightens **13** documents on the trusted tier
+  in vendor-harvest and **14** in generated-notes. Adding the no-URL narrowing takes
+  each of them back to 0: in these populations the document's second, HIGH-class
+  carrier was itself a tag naming no target, which the narrowing makes inert, so the
+  compound overlay never sees two findings. That is the census reporting a
+  cancellation, not this repo proving one — a population whose second carrier is a
+  real `<img src>` would still escalate, which is precisely the case
+  `test_split_tightens_the_trusted_tier_when_both_carriers_are_present` constructs
+  and pins. Read the zero as "not observed in any of the three populations, each
+  counted against its own denominator", never as "cannot happen".
 - **Raw-HTML findings no longer count end tags, and that moved a published field.**
   Through 0.6.1 `</a>` was active by name on its own, so `count` ran roughly 1.6×
   the opening-tag total (measured on one corpus) and a start/end pair counted 2.