fix(active-content): raw-html graded two inert shapes HIGH, and the fix moved a second surface

`is_active_tag`'s URL-attribute branch was a presence test: any element carrying
`href=`/`src=`/`action=` graded HIGH regardless of where the URL pointed. An MDX
`<Card href="/en/agent-sdk/quickstart">` reaches no attacker-controlled host, and
neither does APIM policy XML's `<set-header>`. It now requires an external target
-- the rule the markdown paths have applied since 0.3.1. `<base>` left the active
name set in the same change: HTML's `<base>` has its whole affordance in an `href`
the attribute branch still catches, and APIM's attribute-less `<base />` is inert.

Measured before and after in ONE session against one corpus state, because two of
the three corpora are living and a split would mix this with re-harvest drift:

  reference-corpus  389 docs   133 -> 108   (ceiling 107)
  vendor-harvest    187 docs   100 ->  98   (ceiling  62)
  generated-notes   550 docs    90 ->  88   (ceiling  49)

96% of the achievable reduction in reference-corpus, 5% in the wiki corpora. The
two classes had to be measured TOGETHER -- alone they free 3 and 13 documents,
together 25, because a document carrying one usually carries the other.

The second surface: `neutralize` imported `is_active_tag` by name, so this would
have silently narrowed the opt-in mutator too -- and no test discriminated the two
halves, since every `neutralize:raw-html` payload stays active under any narrowing
considered. That test is written first here. The predicates are now separate
symbols; the mutator keeps defanging anything, because over-defanging is auditable
and blocks nothing while under-defanging hands a human a live construct.

Behaviour change: a document whose only finding was one of these classes now WARNs
instead of holding. Detection is unchanged -- 128/128 classes, 6/6 gaps hold.

Self-safety: reading an attribute VALUE needs a pattern the presence test lacks. It
reuses the same literal alternation so no new run shape enters the table; its
`_REDOS_PAYLOADS` row denies the `=` the pattern requires, since a unit supplying it
matches at once and never exercises the run (the lexicon's `script-tag` row is the
cautionary case). 0.031-0.046s across five attack shapes at 100_000 chars against a
2.0s bound; `docs/redos-sweep.py` reports 0 candidates of 152. An attribute the
presence test saw but the value parser cannot read counts as external -- fail secure.

`docs/rawhtml-census.py` gains a PRODUCTION row that re-measures the shipped
predicate rather than a hypothesis, so a published number and the code cannot drift
apart unnoticed. README's limitation count moves 34 -> 33.

727 passed (was 717).

src/llm_ingestion_guard/neutralize.py

@@ -50,7 +50,7 @@ from .active_content import (
     MD_REFDEF_RE,
     URL_IN_TEXT_RE,
     defang_url,
-    is_active_tag,
+    is_defangable_tag,
     redact,
 )
 from .calibration import MAX_INPUT_CHARS
@@ -146,7 +146,7 @@ def neutralize(
 
     def _html(m: re.Match[str]) -> str:
         tag = m.group(0)
-        if not is_active_tag(m.group("name"), m.group("attrs") or ""):
+        if not is_defangable_tag(m.group("name"), m.group("attrs") or ""):
             return tag
         html_state["count"] += 1
         if not html_state["ev"]: