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/entity_stublib.py

@@ -471,6 +471,11 @@ def load_entity_modules() -> dict:
         "media_player",
         "remote",
         "diagnostics",
+        # t41: button.py was the file nobody loaded — a deleted helper it still
+        # imported shipped silently. It is a platform like the others, so it
+        # belongs here; tests/test_module_coverage.py sweeps EVERY module, which
+        # is what keeps this list from having to be complete.
+        "button",
         "init",
     ):  # "init" maps the integration __init__.py (t29 G8 async_setup)
         if name == "init" and "nsh.init" in sys.modules: