docs(readme): the variant rule is normative in the spec now, and the README still pointed at the manifest

Two loose ends from v0.7.0.

The README sent a reader to case_id_derivation.variant_suffix in the manifest
for the rule about variant cases. That was correct until v0.7.0 made the rule
normative in spec section 6 and left the manifest block as the MEASUREMENT
behind it. Left alone it reproduces in one line the defect v0.7.0 closed: a
reader sent to the wrong authority. Both are named now, each for what it is.

And the release that made a point of re-measuring '--' rather than copying the
manifest's 0.3.0 numbers forward had inherited the adjacent '__' claim
untested. Measured now across all five published id spaces - 83 lexicon ids, 7
active: ids, 3 carrier: ids, 7 malware rule ids, 19 secret-egress entry names -
neither '__' nor '--' occurs in any of them. The one-to-one transform holds, so
no text changed; the sentence was true and is now measured.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01M4xrxV3EXbSALqvB23kpeY

README.md

@@ -93,8 +93,9 @@ The corpus covers three tables, and they do not carry equal weight — treating
 number would misreport all three:
 
 - `lexicon/injection-lexicon.json` — 84 cases over 83 patterns. Both seeding runtimes
-  implement it and both ratified its id space. One pattern carries a second, variant case;
-  see `case_id_derivation.variant_suffix` in the manifest.
+  implement it and both ratified its id space. One pattern carries a second, variant case:
+  the rule for when that is legal is normative in [§6](spec/conformance-corpus.md), and
+  `case_id_derivation.variant_suffix` in the manifest carries the measurement behind it.
 - `signatures/active-content.json` — 7 cases, one per published id, the seventh added in
   v0.6.0 when the seed runtime split raw HTML into two carrier classes. One runtime
   implements it. For a runtime that