fix(885): the guard states both invariants, and the second one is executed

The two `docs/guard-inventory.md` rows and the module docstring described
`test_workflow_persist_credentials.py` as the `actions/checkout` guard only. The
deferral to #909 rested on `docs/guard-inventory.md` being held by the session
working #881; that issue is closed and its PR is the commit this branch is rebased
onto, so the file is free and the edit belongs here under docs-update-is-part-of-done.

`MUTATIONS` keys at most one declared clause mutation per guard FILE
(`test_the_manifest_covers_exactly_the_MUTATION_rows` asserts `len(MUTATIONS) ==
len(declared)`), and the grading row's proof-ref column is compared against it, so
the route invariant cannot take a second `MUTATIONS` row. It takes a `CLAIMS` entry
instead — the population #881 widened this file to carry — bound to the inventory
sentence that states it: deleting `build`'s `if: github.event_name != 'pull_request'`
from the shipped `docker-build.yml` is applied to a sandbox copy every run and the
named proof is required to redden with the collector's own wording.

That grows the `CLAIMS` population from three entries to four, which invalidates the
cost span `testing.mutation-claims-are-executed` measured over three. Re-taking it
here produced 54.3s/149.5s, 81.6s/78.7s and 114.3s/84.2s across three A/B pairs with
other builds on the host — two inverted, so the load dominates the signal. The record
now says the span is a lower bound and that a re-measurement is owed on a quiet
machine, rather than carrying a scaled or invented number.

Decisions-Edit: yes
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015QqCpYFsKgnAnx6jVwrKiV

scripts/tests/test_workflow_persist_credentials.py

@@ -1,4 +1,9 @@
-"""Every `actions/checkout` drops the persisted credential (ersatztv#835).
+"""TWO credential invariants over one derived workflow population.
+
+Every `actions/checkout` drops the persisted credential (ersatztv#835) — below — and no job
+the `pull_request` trigger reaches, nor the workflow scope outside `jobs:`, names a stored
+secret (ersatztv#885). The second is documented at its own banner further down, where the
+population it adds to this one is derived; what follows here is the first.
 
 WHAT THIS IS PROTECTING. `actions/checkout` writes an `Authorization` header into `.git/config`
 unless `persist-credentials: false` is set, and this instance's Actions default token permission is