feat: refuse matrix settings a Raspberry Pi 5 can't drive

On a Pi 5 the pinned rgbmatrix library drives the panel through the RP1
chip, and that path supports only row address types 0 and 2, parallel 1-3
and the regular / regular-pi1 / classic / adafruit-hat(-pwm) mappings
(Rp1PioConfigSupported in lib/rp1/rp1_pio_backend.cc). For anything else
CreateFromOptions returns NULL; the Python binding doesn't check, so the
display process crashed on its first call into the matrix and systemd
restarted it into the same crash every 10 seconds.

- src/pi5_matrix_support.py: the rule and Pi 5 detection, matching the
  library's /proc/device-tree/model check
- DisplayManager raises before creating the matrix, so it is a logged init
  failure (reported by /api/v3/hardware/status) and fallback mode
- the config API rejects those settings on a Pi 5 when a request sets
  row_address_type, parallel or hardware_mapping
- the Display form offers only row address types 0 and 2 on a Pi 5, and
  warns when a stored value can't be used
- CLAUDE.md: re-check the rule whenever the submodule is bumped

Review feedback on #586.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

test/test_pi5_matrix_support.py

@@ -0,0 +1,79 @@
+"""The Pi 5 rule in src/pi5_matrix_support.py mirrors the pinned library.
+
+Rp1PioPlatformDetected() and Rp1PioConfigSupported() in
+rpi-rgb-led-matrix-master/lib/rp1/rp1_pio_backend.cc (commit 1ee4f76). When the
+submodule is bumped and those change, these are the cases to revisit.
+"""
+import sys
+from pathlib import Path
+
+import pytest
+
+sys.path.insert(0, str(Path(__file__).parent.parent))
+
+from src import pi5_matrix_support as pi5  # noqa: E402
+
+
+@pytest.fixture
+def model_file(tmp_path, monkeypatch):
+    def write(model):
+        path = tmp_path / 'model'
+        # The device tree string is NUL-terminated.
+        path.write_bytes(model.encode() + b'\x00')
+        monkeypatch.setattr(pi5, 'MODEL_PATH', str(path))
+    return write
+
+
+@pytest.mark.parametrize('model', [
+    'Raspberry Pi 5 Model B Rev 1.0',
+    'Raspberry Pi 500 Rev 1.0',
+    'Raspberry Pi Compute Module 5 Rev 1.0',
+])
+def test_pi5_family_is_detected(model_file, model):
+    model_file(model)
+    assert pi5.is_raspberry_pi_5()
+
+
+@pytest.mark.parametrize('model', [
+    'Raspberry Pi 4 Model B Rev 1.5',
+    'Raspberry Pi Zero 2 W Rev 1.0',
+    'Raspberry Pi Compute Module 4 Rev 1.0',
+])
+def test_other_boards_are_not(model_file, model):
+    model_file(model)
+    assert not pi5.is_raspberry_pi_5()
+
+
+def test_no_device_tree_is_not_a_pi5(tmp_path, monkeypatch):
+    monkeypatch.setattr(pi5, 'MODEL_PATH', str(tmp_path / 'missing'))
+    assert not pi5.is_raspberry_pi_5()
+
+
+@pytest.mark.parametrize('hardware', [
+    {},
+    {'row_address_type': 0}, {'row_address_type': 2}, {'row_address_type': '2'},
+    {'parallel': 1}, {'parallel': 3},
+    {'hardware_mapping': 'regular'}, {'hardware_mapping': 'regular-pi1'},
+    {'hardware_mapping': 'classic'}, {'hardware_mapping': 'adafruit-hat'},
+    {'hardware_mapping': 'adafruit-hat-pwm'}, {'hardware_mapping': ''},
+])
+def test_what_the_pi5_path_supports(hardware):
+    assert pi5.pi5_unsupported_settings(hardware) is None
+
+
+@pytest.mark.parametrize('hardware,named', [
+    ({'row_address_type': 1}, 'row address type 1'),
+    ({'row_address_type': 3}, 'row address type 3'),
+    ({'row_address_type': 4}, 'row address type 4'),
+    ({'row_address_type': '5'}, 'row address type 5'),
+    ({'parallel': 4}, 'parallel 4'),
+    ({'hardware_mapping': 'compute-module'}, 'hardware mapping "compute-module"'),
+])
+def test_what_it_does_not(hardware, named):
+    message = pi5.pi5_unsupported_settings(hardware)
+    assert message is not None and named in message
+
+
+def test_every_problem_is_named():
+    message = pi5.pi5_unsupported_settings({'row_address_type': 5, 'parallel': 4})
+    assert 'row address type 5' in message and 'parallel 4' in message