test: isolate DisplayManager globals across modules, and name the failure (#563)

Follow-up to #562. That commit fixed the actual cause of the intermittent
15-test failure in test_display_dirty_tracking.py -- the emulator's fixed TCP
port 8888, a machine-wide singleton that a concurrent pytest process takes
away. This adds the two things that would have made it a five-minute
diagnosis instead of a long one, and closes the other door into the same
failure.

Confirmed the module is order-independent as it stands, on this checkout:

  pytest test/ -q, three times          115 failed / 4464 passed / 63 skipped,
                                        byte-identical failure sets, the
                                        module 21/21 passed each time
  module forced last (197 files first)  identical failure set
  module forced first                   identical failure set
  module after each of test_display_manager, test_display_controller,
    test_display_controller_vegas_tick, test_skin_system, test_sports_scroll,
    test_initial_update_budget, test_display_double_parity,
    test_initializing_screen                                     all pass
  four concurrent processes on the file                          21/21 each

And reproduced the original, to be sure the diagnosis in #562 is the whole
story. Holding 0.0.0.0:8888 from a separate process:

  HEAD's test/conftest.py        21 passed
  pre-#562 test/conftest.py      15 failed, 6 passed

The 15/6 split is not arbitrary: the six survivors are the only tests in the
file that never touch dm.matrix.

conftest.py: DisplayManager is a process-wide singleton and the RGBMatrix /
RGBMatrixOptions names it constructs through are module globals, bound once at
import. All three are shared by every test module in the run, so a module that
leaves an instance in _instance -- or leaves patch('src.display_manager.
RGBMatrix') standing -- changes what the NEXT module builds, invisibly, and
only in a full run. A module-scoped autouse fixture now resets the singleton
and restores either binding if a patch outlived its module. Module-scoped
rather than per-test so that files sharing one manager across their own tests
keep doing so; only the leak across the module boundary is cut. Autouse
fixtures are set up ahead of requested ones, so this is finalised after a
module's own DisplayManager fixture. Verified with a throwaway pair of probe
modules -- one leaks a patch and a singleton, the next asserts both are clean
-- which passed and were then removed.

test_display_dirty_tracking.py: _setup_matrix() swallows every construction
failure and falls back to matrix=None, so a broken environment arrived as
fifteen identical "'NoneType' object has no attribute 'SwapOnVSync'" errors
naming neither the fixture nor the cause. The fixture now fails once, and
says where to look; under a held port it reads

    DisplayManager fell back to matrix=None: RGBMatrix construction raised...
    Known causes: the emulator adapter losing a fixed TCP port to another
    process -- see pytest_configure in test/conftest.py -- or a
    patch('src.display_manager.RGBMatrix') leaked from an earlier test module.

with WinError 10048 in the captured log directly above it.

No regressions: full suite with both changes is 115 failed / 4464 passed /
63 skipped, failure set identical to the pre-change baseline. The 115 is the
pre-existing Windows-environment baseline (os.geteuid, POSIX modes, fcntl);
CI on Linux remains authoritative.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

test/test_display_dirty_tracking.py

@@ -44,6 +44,20 @@ def dm(tmp_path_factory):
     # this session; the individual tests that care still override it further.
     manager._snapshot_path = str(
         tmp_path_factory.mktemp("dirty_tracking") / "led_matrix_preview.png")
+    # _setup_matrix() swallows every construction failure and falls back to
+    # matrix=None, so a broken environment reaches the tests as fifteen
+    # identical "'NoneType' object has no attribute 'SwapOnVSync'" errors that
+    # name neither this fixture nor the real cause. Fail here instead, once,
+    # and say where to look.
+    if manager.matrix is None:
+        pytest.fail(
+            "DisplayManager fell back to matrix=None: RGBMatrix construction "
+            "raised (the 'Failed to initialize RGB Matrix' log line above "
+            "carries the reason). Known causes: the emulator adapter losing a "
+            "fixed TCP port to another process -- see pytest_configure in "
+            "test/conftest.py, which pins the port-free 'raw' adapter -- or a "
+            "patch('src.display_manager.RGBMatrix') leaked from an earlier "
+            "test module.")
     yield manager
     DisplayManager._instance = None
     DisplayManager._initialized = False