fix(#3841): honor the payload anchor in the classifier, dedup the fixture

Three review findings, all fixed.

The isolated security review found a SECOND divergence the design missed. The
classifier parses structurally, so it accepted `packageName` at any key
position; the shell's `case` is anchored at the start of stdout. For
`{"note":"x","packageName":"<us>",...}` the classifier said ok and the shell
said unverified -- a fail-open disagreement, and none of the original ten parity
rows caught it because every one put `packageName` first. Certifying agreement
that does not hold would have been worse than shipping no parity suite. The
classifier now honors the anchor on its ok arm, which is what the module already
claimed to do: IDENTITY_RAW_PREFIX is documented as "ANCHORED, never a substring
search". A foreign packageName stays identity_mismatch wherever it appears, so
that arm is untouched. Parity rows P11-P13 added.

The spec review found the test matrix marked the empty-string `packageName` row
as already covered. It was not -- the `.length > 0` guard is a distinct path
from "no packageName key at all", which is tested. Added, and the matrix
corrected to say it was wrong.

The standards review flagged ~45 lines of fixture helpers duplicated verbatim
between the two preamble describes. Extracted to one `makeIdentityFixture()`.

Changeset rewritten: it described only the exit-code half of the diff.

Refs #3841

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.changeset/sturdy-jaguars-munch.md

@@ -2,4 +2,4 @@
 type: Fixed
 pr: 0
 ---
-**The identity classifier now agrees with the launcher preamble about a tool that proves itself and then exits non-zero** — the two surfaces implement one decision and disagreed on exactly that input, so the announced hard-fail rollout would have refused an install the current warn phase verifies. (#3841)
+**The identity classifier and the launcher preamble now reach the same verdict for the same probe** — the two surfaces implement one decision and disagreed on two inputs, a tool that proves itself and then exits non-zero and a payload naming this package outside the anchored wire shape, so the announced hard-fail rollout would have refused installs the warn phase verifies and accepted ones it warns about. (#3841)