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>

web_interface/blueprints/api_v3/config.py

@@ -12,6 +12,7 @@ from web_interface.blueprints.api_v3 import (
     success_response,
 )
 from src.common.path_safety import resolve_under
+from src.pi5_matrix_support import is_raspberry_pi_5, pi5_unsupported_settings
 import web_interface.blueprints.api_v3 as _pkg
 # Read through the module rather than bound by value: tests patch these
 # as module attributes, and a value binding would not see the patch.
@@ -606,6 +607,18 @@ def save_main_config():
                     if error:
                         return error
 
+            # A Pi 5 can't drive every combination (src/pi5_matrix_support.py),
+            # and one it can't crashes the display service instead of falling
+            # back. Checked only when this request sets one of those fields, so
+            # a combination already stored doesn't block unrelated saves.
+            pi5_fields = ('row_address_type', 'parallel', 'hardware_mapping')
+            if any(k in data for k in pi5_fields) and is_raspberry_pi_5():
+                effective = dict(current_config['display']['hardware'])
+                effective.update({k: data[k] for k in pi5_fields if k in data})
+                unsupported = pi5_unsupported_settings(effective)
+                if unsupported:
+                    return jsonify({'status': 'error', 'message': unsupported}), 400
+
             # Handle hardware settings
             for field in ['rows', 'cols', 'chain_length', 'parallel', 'brightness', 'hardware_mapping', 'scan_mode',
                          'pwm_bits', 'pwm_dither_bits', 'pwm_lsb_nanoseconds', 'limit_refresh_rate_hz',