ci: run the whole test tree and make the plugin-safety job assert something real

The unit-tests CI job ran an explicit 24-file allowlist that had rotted:
63 of 90 test files (display, vegas, store manager, web API, web_interface)
never ran on a PR. The job now runs all of test/ (minus test/plugins, which
the plugin-safety job owns) so new test files are enrolled by default and
any exclusion needs a visible, commented --ignore.

The plugin-safety job was a green no-op: plugins/ is empty in CI, so every
test skipped with 'Manifest not found'. It now renders a bundled
deterministic fixture plugin (test/fixtures/plugins/ci-fixture-plugin,
golden images included for all 8 default sizes) via LEDMATRIX_PLUGINS_DIR,
and sets LEDMATRIX_REQUIRE_PLUGINS=1 so discovering zero plugins fails
loudly instead of skipping green. The per-plugin suites document that they
target dev machines with real plugins installed.

Coverage is now measured and enforced in exactly one place — the CI
unit-tests step (--cov=src --cov=web_interface --cov-fail-under=45, from a
measured 47% baseline). pytest.ini previously declared --cov-fail-under=30
but CI always passed --no-cov, so the gate had never run anywhere; local
pytest is now coverage-free and fast.

Enabling the 63 unenrolled files surfaced three cases of test rot, fixed
here: test_display_controller_vegas_tick.py could not collect without the
hardware rgbmatrix module (now uses the emulator convention), the
state-reconciliation unrecoverable-cache tests broke when production added
the is_plugin_uninstalled tombstone check (bare Mock returned truthy),
and test_get_system_status assumed the optional psutil dependency
(now installed via requirements-test.txt and guarded by importorskip).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NohXi78cwsAKtN1sCfxjUh

test/plugins/conftest.py

@@ -23,9 +23,17 @@ os.environ['EMULATOR'] = 'true'
 def plugins_dir() -> Path:
     """Get the plugins directory path.
 
-    Checks plugins/ first, then falls back to plugin-repos/
-    for monorepo development environments.
+    Honors LEDMATRIX_PLUGINS_DIR (first entry) when set — the same override
+    test_plugin_matrix.py uses, so CI can point every plugin suite at the
+    bundled fixture plugins. Otherwise checks plugins/ first, then falls
+    back to plugin-repos/ for monorepo development environments.
     """
+    env = os.environ.get('LEDMATRIX_PLUGINS_DIR')
+    if env:
+        first = env.split(os.pathsep)[0]
+        if first:
+            return Path(first)
+
     plugins_path = project_root / 'plugins'
     plugin_repos_path = project_root / 'plugin-repos'