feat(active-content): raw HTML graded on carrier, and a tag naming no target is inert

Two changes that had to ship together, because they co-occur.

`active:raw-html-link` (MEDIUM) splits the click-required carriers out of
`active:raw-html`. The same URL was LOW as `[t](url)` and HIGH as
`<a href="url">` — an asymmetry produced by syntax, not by affordance, on a
carrier the markdown path has graded MEDIUM since 0.3.1. The event-handler test
runs first, so `<a onclick=...>` stays HIGH. The url-attribute branch stays HIGH
too: a name outside the active set has unknown rendering, and grading
`<Card src=...>` as a link would be reasoning rather than measurement.

The no-URL narrowing makes `</a>`, `<Frame>`, `<video />` and `<img alt=...>`
without `src` inert — `<base />`'s argument from 0.6.0 applied to the rest of the
name branch. It tests for the URL attribute's PRESENCE, not for a readable value,
so the fail-secure gap `_url_attr_is_external` leaves open is not reopened here.

WHY TOGETHER: the narrowing strips a document's `</a>`/`<Frame>` and what remains
is the `<a href=...>` the split grades down, so each alone leaves the document
blocked by the other's residue.

`active_tag_class` is now the classification point and `is_active_tag` wraps it.
The census patches the former: a boolean could only express a narrowing, never a
regrade, so every carrier candidate would have measured equal to PRODUCTION —
silently, and in the direction that reads as "no change helps".

TWO COSTS, BOTH RECORDED RATHER THAN GLOSSED:

- The split TIGHTENS the trusted tier. One finding becomes two, and >=2 findings
  at MEDIUM+ trip the compound overlay, so a document carrying both an `<img src>`
  and an `<a href>` goes WARN -> quarantine_review on PRESET_TRUSTED_SOURCE. On
  that preset it is the only direction the split can move anything. The census
  now reports a TIGHTENS column on both trust tiers against the previously
  shipped row — "frees N" without "tightens M" is a one-sided number.
- `count` drops on documents containing `</a>`, a published field moving under a
  meaning that did not change.

MEASURED: reference-corpus (389) 54 -> 53 fail_secure, tightens 0/0, and the
census `PRODUCTION` row equals its `C1 + D` candidate row for row. The census
also reproduces 133/3/13/108/25 exactly, so it is calibrated against every
published historical number. The two wiki corpora are NOT yet re-measured; the
tree says so explicitly in the docstring, LIMITATIONS and CHANGELOG rather than
carrying probe numbers as fact.

791 tests (was 759), coverage 129/129, 6/6 documented gaps holding. Version
bumped to 0.7.0 across every surface; no tag is set until the measurement lands.

tests/test_neutralize.py

@@ -102,6 +102,13 @@ def test_raw_active_html_is_escaped():
 @pytest.mark.parametrize("cid,text", [
     ("relative-href-on-inactive-name", '<Card href="/en/agent-sdk/quickstart">'),
     ("attributeless-base", "<base />"),
+    # 0.7.0's no-URL narrowing. The scanner now lets these pass — they name no
+    # target — but the mutator still escapes them, because a human auditing
+    # defanged output should see the markup that was there.
+    ("end-tag", "</a>"),
+    ("mdx-wrapper-component", "<Frame>"),
+    ("self-closing-media", "<video />"),
+    ("img-without-src", '<img alt="a diagram">'),
 ])
 def test_mutator_still_defangs_what_the_scanner_now_lets_pass(cid, text):
     # The deliberate asymmetry, extended to raw HTML in 0.6.0: the SCANNER narrowed