Add PlatformIO CI build and README badge

GitHub Actions workflow compiles the XIAO firmware with `pio run` on
push/PR; README badge reflects its status.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: hachary-Brown86

.github/workflows/build.yml

@@ -0,0 +1,31 @@
+name: PlatformIO Build
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+  workflow_dispatch:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Cache PlatformIO
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.platformio
+            ~/.cache/pip
+          key: ${{ runner.os }}-pio-${{ hashFiles('platformio.ini') }}
+
+      - name: Install PlatformIO
+        run: pip install platformio
+
+      - name: Build firmware
+        run: pio run

README.md

@@ -1,5 +1,6 @@
 # mesh-cam
 
+[![PlatformIO Build](https://github.com/stackzac22/mesh-cam/actions/workflows/build.yml/badge.svg)](https://github.com/stackzac22/mesh-cam/actions/workflows/build.yml)
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](LICENSE)
 
 A two-board off-grid camera node for a Meshtastic mesh.