fix(pi5): auto-detect Pi 5 and force rgbmatrix rebuild when rp1_rio missing

first_time_install.sh:
- Detect Pi 5 from /proc/device-tree/model at startup
- Step 6 skip logic now also checks hasattr(RGBMatrixOptions(), 'rp1_rio'):
  if the installed library lacks rp1_rio (built before Pi 5 support was added)
  the build is forced even when the module is already importable. This is the
  root cause of mmap errors to 0x3f000000 (Pi 3 bus) on Pi 5 hardware.
- After a successful Pi 5 build, verify rp1_rio is present and print a
  diagnostic with the submodule update command if it's still missing.

src/display_manager.py:
- rp1_rio warning now names the symptom (mmap to 0x3f000000) and gives the
  exact fix command so users can act immediately from the log.

README.md:
- Remove "Pi 5 is unsupported" — Pi 5 is fully supported since the library
  submodule includes rp1_pio and rp1_rio backends.
- Document the forced-rebuild command for users migrating from Pi 4.
- Fix gpio_slowdown guidance: Pi 5 PIO mode uses 1–2, not 5.

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

first_time_install.sh

@@ -36,9 +36,17 @@ if [ -r /proc/device-tree/model ]; then
     DEVICE_MODEL=$(tr -d '\0' </proc/device-tree/model)
     echo "Detected device: $DEVICE_MODEL"
 else
+    DEVICE_MODEL=""
     echo "⚠ Could not detect Raspberry Pi model (continuing anyway)"
 fi
 
+# Detect Pi 5 for hardware-specific install decisions (RP1 library verification)
+IS_PI5=0
+if echo "${DEVICE_MODEL:-}" | grep -qi "Raspberry Pi 5"; then
+    IS_PI5=1
+    echo "Raspberry Pi 5 detected — will verify RP1 library support."
+fi
+
 # Check OS version - must be Raspberry Pi OS Lite (Trixie)
 echo ""
 echo "Checking operating system requirements..."
@@ -783,9 +791,26 @@ CURRENT_STEP="Build and install rpi-rgb-led-matrix"
 echo "Step 6: Building and installing rpi-rgb-led-matrix..."
 echo "-----------------------------------------------------"
 
-# If already installed and not forcing rebuild, skip expensive build
+# On Pi 5, also check that the installed library has rp1_rio support.
+# A library built before Pi 5 support was added imports fine but maps to the
+# Pi 3 peripheral bus address (0x3f000000) instead of the RP1 chip at runtime.
+_HAS_RP1=0
+if python3 -c 'from rgbmatrix import RGBMatrixOptions; assert hasattr(RGBMatrixOptions(), "rp1_rio")' >/dev/null 2>&1; then
+    _HAS_RP1=1
+fi
+
+_SKIP_BUILD=0
 if python3 -c 'from rgbmatrix import RGBMatrix, RGBMatrixOptions' >/dev/null 2>&1 && [ "${RPI_RGB_FORCE_REBUILD:-0}" != "1" ]; then
-    echo "rgbmatrix Python package already available; skipping build (set RPI_RGB_FORCE_REBUILD=1 to force rebuild)."
+    if [ "$IS_PI5" = "1" ] && [ "$_HAS_RP1" = "0" ]; then
+        echo "⚠ Pi 5 detected: installed rgbmatrix lacks rp1_rio support (older build)."
+        echo "  Forcing rebuild to get Pi 5 RP1 support..."
+    else
+        _SKIP_BUILD=1
+    fi
+fi
+
+if [ "$_SKIP_BUILD" = "1" ]; then
+    echo "rgbmatrix already installed${IS_PI5:+ with Pi 5 RP1 support}; skipping build (set RPI_RGB_FORCE_REBUILD=1 to force rebuild)."
 else
     # Ensure rpi-rgb-led-matrix submodule is initialized
     if [ ! -d "$PROJECT_ROOT_DIR/rpi-rgb-led-matrix-master" ]; then
@@ -852,6 +877,17 @@ except Exception as e:
 PY
     then
         echo "✓ rpi-rgb-led-matrix installed and verified"
+        # Pi 5: confirm the freshly-built library has rp1_rio support
+        if [ "$IS_PI5" = "1" ]; then
+            if python3 -c 'from rgbmatrix import RGBMatrixOptions; assert hasattr(RGBMatrixOptions(), "rp1_rio")' >/dev/null 2>&1; then
+                echo "✓ Pi 5 RP1 (rp1_rio) support confirmed"
+            else
+                echo "⚠ rp1_rio not found after rebuild — the submodule may be an older version."
+                echo "  Try updating the submodule and rebuilding:"
+                echo "    git submodule update --remote rpi-rgb-led-matrix-master"
+                echo "    sudo RPI_RGB_FORCE_REBUILD=1 ./first_time_install.sh"
+            fi
+        fi
     else
         echo "✗ rpi-rgb-led-matrix import test failed"
         exit 1