CI-12: CI grades test_freeze.py against the real hwsim at a pinned admin/goodwe

- .gitea/infra-pin: the one place the admin/goodwe commit is pinned.
- checks.yml: checks out admin/goodwe at the pin as the sibling test_freeze.py
  looks for; any failure fails the job. Tests run with GOODWE_REQUIRE_HWSIM=1,
  and `CI: ` lines are echoed on PASS so the log shows which transport ran.
- test_freeze.py: GOODWE_REQUIRE_HWSIM refuses the stub; a local stub run
  prints a STUB USED line.
- infra-pin.yml: a separate, non-required check that goes red naming both
  SHAs when admin/goodwe master has moved past the pin.
- README: when and how to bump the pin.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -105,6 +105,11 @@ has the mechanism, the measurement, and what the flags do not do. They are not u
 each assertion corresponds to a rule whose absence produced an observed failure
 on real hardware, and the comments in `control.py` say which.
 
+### CI and the infra pin
+
+CI (`.gitea/workflows/checks.yml`) runs every `test_*.py`, with `admin/goodwe` checked out next to this repo at the commit in `.gitea/infra-pin`, so `test_freeze.py` grades against the real `sim/hwsim.py`. If that checkout fails, the job fails. The local stub is used only in local runs, and it prints `STUB USED` when it is.
+**Bump the pin** whenever `infra-pin / fresh` is red, which happens once `admin/goodwe` master has moved past the pin. Set `.gitea/infra-pin` to the new master SHA (`git ls-remote <gitea>/admin/goodwe.git refs/heads/master`) in a PR, and let `checks / unit` grade the tests against it.
+
 ## Compatibility
 
 **Check the serial number, not the model name.** Characters 6-8 of the serial are