feat: add HIL diagnostics and meter health handling

hil_tests/README.md

@@ -0,0 +1,15 @@
+# Local HIL tests
+
+The Python unit tests validate trace parsing, simulator fixtures, and strict evidence correlation without opening hardware. The `hil`-marked test reads an already completed artifact directory; pytest itself never opens or flashes a serial device.
+
+```bash
+python3 -m venv .venv-hil
+.venv-hil/bin/pip install -r requirements-hil.txt
+.venv-hil/bin/pytest -v
+HIL_ARTIFACT=artifacts/hil/<timestamp> .venv-hil/bin/pytest -v -m hil hil_tests/
+HIL_ARTIFACT=artifacts/hil/<fault-timestamp> HIL_FAULT=suppress_ack .venv-hil/bin/pytest -v -m hil hil_tests/
+```
+
+A missing real meter is represented as `BLOCKED`/JUnit `skipped`, not as a firmware failure. A missing boot, role, time-sync, batch, decode, or ACK event is a failure because there is no corresponding device evidence.
+
+Live serial access is performed only by the explicitly invoked tools in `tools/hil/`, after `.hil/local.toml` approval checks.