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/fixtures/plugins/ci-fixture-plugin/requirements.txt

@@ -0,0 +1 @@
+# No dependencies — the fixture must load in any environment.