ci: add windows, linux arm, and macos jobs to the test matrix

Same platform coverage as SmartString: PHP 8.1-8.5 stays on ubuntu x64,
plus one PHP 8.1 job each on windows x64, linux arm64, macos arm64, and
macos intel. No windows arm job: php.net ships no arm64 windows builds.

Author: daveedis

.github/workflows/tests.yml

@@ -15,13 +15,26 @@ on:
 
 jobs:
   test:
-    name: PHP ${{ matrix.php }}
-    runs-on: ubuntu-latest
+    name: PHP ${{ matrix.php }} on ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
 
     strategy:
       fail-fast: false
       matrix:
+        os: [ubuntu-latest]
         php: ${{ inputs.php && fromJSON(format('["{0}"]', inputs.php)) || fromJSON('["8.1","8.2","8.3","8.4","8.5"]') }}
+        include:
+          # OS/arch bugs (paths, case-insensitive filenames, char signedness in
+          # extensions) don't vary by PHP version, so one job per platform at the
+          # 8.1 floor. No Windows ARM job: php.net ships no ARM64 Windows builds.
+          - { os: windows-latest,    php: '8.1' }   # Windows x64 - CMSB production
+          - { os: ubuntu-24.04-arm,  php: '8.1' }   # Linux arm64 - Graviton-class hosts
+          - { os: macos-latest,      php: '8.1' }   # macOS arm64 - Apple Silicon devs
+          - { os: macos-15-intel,    php: '8.1' }   # macOS x64 - Intel Macs; drop after a quiet stretch
+
+    defaults:
+      run:
+        shell: bash
 
     steps:
       - name: Checkout