chore(deps): update rpi-rgb-led-matrix to latest upstream for Pi 5 support

Configure submodule to track upstream master branch (branch = master in
.gitmodules) so future updates are a single 'git submodule update --remote'
rather than manual SHA management.

Update first_time_install.sh to use --remote flag so fresh installs always
pull the current upstream master, not the commit recorded at clone time.

Current upstream HEAD (8907235) brings:
- PR #1886: Raspberry Pi 5 support — new RP1 PIO and RIO backends. The
  library auto-detects Pi 5 hardware at runtime; no config change required
  for basic operation. adafruit-hat-pwm is confirmed supported on Pi 5.
- PR #1833: setup.py migrated from distutils → setuptools, fixing Python
  3.12+ build failure (Pi runs Python 3.13). Previous version could not
  build the bindings at all on current Pi OS.

Expose new rp1_rio option in display_manager.py and config.template.json:
  0 (default) = PIO mode — uses Pi 5 RP1 coprocessor, minimal CPU usage
  1 = RIO mode — Registered IO, faster throughput, higher CPU; note that
      gpio_slowdown has inverted effect in this mode

No API changes to RGBMatrix, RGBMatrixOptions, or FrameCanvas. Pi 4 and
earlier hardware is unaffected — rp1_rio is silently ignored on non-Pi-5.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/display_manager.py

@@ -100,6 +100,10 @@ class DisplayManager:
                 options.pwm_dither_bits = hardware_config.get('pwm_dither_bits')
             if 'inverse_colors' in hardware_config:
                 options.inverse_colors = hardware_config.get('inverse_colors')
+            # Pi 5 only: 0=PIO/RP1 coprocessor (default, less CPU),
+            # 1=RIO/Registered IO (faster; gpio_slowdown effect is inverted in this mode)
+            if 'rp1_rio' in runtime_config:
+                options.rp1_rio = runtime_config.get('rp1_rio')
             
             logger.info(f"Initializing RGB Matrix with settings: rows={options.rows}, cols={options.cols}, chain_length={options.chain_length}, parallel={options.parallel}, hardware_mapping={options.hardware_mapping}")