test(ci): sweep every module for dangling imports (t41)

Two defects shipped with a green suite: button.py imported a helper that had
been deleted (no harness ever loaded the file), and the drift sweep only judged
`homeassistant.*`, so a relative import of a name that no longer exists was
invisible to it. Both are now impossible to ship silently.

* tests/test_module_coverage.py loads EVERY *.py file of the package, in a clean
  interpreter, under the stub harness — exhaustive by construction (a new
  platform is covered the moment the file exists; there is no list and no
  special case). It asserts the reported set equals the files on disk, so a
  silently skipped module fails too;
* entity_stublib.load_entity_modules() now also loads button.py (the named gap);
  the sweep is what keeps that list from needing to be complete;
* ha_api_drift gained pass 5: the package's own relative imports. Pure AST,
  no homeassistant required, with its own entry point
  `scan_package_imports()` for the fast suite — it checks the target file exists
  and defines every imported name (submodule imports still resolve), instead of
  trusting that the modules a harness happened to load are the ones that matter;
* the file walk in both AST passes is recursive, and the non-vacuity floors moved
  with it (modules 10 → 20, relative_imports ≥ 100 — a shallow-glob regression
  trips the floor instead of looking green);
* two permanent red proofs, both on a copy of the tree with the reverted
  `from .helpers import discover_adb_host` reference in button.py: the stub-harness
  sweep fails with "FAIL button.py", and the static pass fails with
  "button.py:1: broken-relative-import: … not defined in helpers.py". Manual runs
  confirm the engine's other direction on the real pre-fix tree (c588a7c: exit 1
  with the t40 entity-contract finding) and exit 0 on HEAD.

No Home-Assistant API drift (23 modules, 58 imports, 148 relative imports, 36
call arguments, 6 awaited HA calls). Verified: 334 passed / 2 skipped, 335 / 1
with androidtvremote2, ruff clean.

Note (pre-existing, not introduced here): running a SUBSET of test files in an
order the suite was not written for (e.g. test_init.py alone after
test_standalone_setup.py) fails 3 import-step tests with
"ModuleNotFoundError: No module named 'voluptuous'" — the per-file stub scoping
assumes the suite's order. Verified identical on the pre-t41 tree (60d884b), so
it is left alone here.

tests/test_entity_contract.py

@@ -108,11 +108,17 @@ def test_the_sweep_reports_the_same_findings_as_this_check(tmp_path):
     report = ha_api_drift.scan(tmp_path)
     kinds = [finding.kind for finding in report.findings]
 
-    # The contract pass runs FIRST (pure AST). Without homeassistant installed the
-    # sweep then adds "cannot judge the API surface" — that path is covered in
-    # tests/test_ha_api_drift.py.
+    # The contract pass runs FIRST (pure AST). What the API passes add after it
+    # depends on the interpreter: with homeassistant installed they judge the
+    # module's imports, without it the sweep says it cannot judge (both are
+    # covered here so the check holds in either variant).
     assert kinds[0] == "entity-attribute-is-a-method"
-    assert "no-homeassistant" in kinds
+    rest = set(kinds[1:])
+    if ha_api_drift.homeassistant_available():
+        # the API passes ran and found nothing wrong with this module
+        assert rest == set(), rest
+    else:
+        assert "no-homeassistant" in rest
 
 
 def test_the_check_ignores_missing_paths_instead_of_raising():