ci(actions): add composite actions and clang-tidy PR helper

Add four reusable building blocks that upcoming CI optimization PRs will
consume. No existing workflow is modified; these files are dormant until
referenced.

- .github/actions/setup-ccache: restore ~/.ccache with content-hash keys,
  write ccache.conf with compression and content-based compiler check
- .github/actions/save-ccache: print stats and save the cache under the
  primary key produced by setup-ccache
- .github/actions/build-gazebo-sitl: build px4_sitl_default plus the
  Gazebo Classic plugins with ccache stats between stages
- Tools/ci/run-clang-tidy-pr.py: compute the translation units affected
  by a PR diff and invoke Tools/run-clang-tidy.py on that subset only,
  exiting silently when no C++ files changed

Signed-off-by: Ramon Roche <mrpollo@gmail.com>

.github/actions/build-gazebo-sitl/action.yml

@@ -0,0 +1,21 @@
+name: Build Gazebo Classic SITL
+description: Build PX4 firmware and Gazebo Classic plugins with ccache stats
+
+runs:
+  using: composite
+  steps:
+    - name: Build - PX4 Firmware (SITL)
+      shell: bash
+      run: make px4_sitl_default
+
+    - name: Cache - Stats after PX4 Firmware
+      shell: bash
+      run: ccache -s
+
+    - name: Build - Gazebo Classic Plugins
+      shell: bash
+      run: make px4_sitl_default sitl_gazebo-classic
+
+    - name: Cache - Stats after Gazebo Plugins
+      shell: bash
+      run: ccache -s

.github/actions/save-ccache/action.yml

@@ -0,0 +1,22 @@
+name: Save ccache
+description: Print ccache stats and save to cache
+
+inputs:
+  cache-primary-key:
+    description: Primary cache key from setup-ccache output
+    required: true
+
+runs:
+  using: composite
+  steps:
+    - name: Cache - Stats
+      if: always()
+      shell: bash
+      run: ccache -s
+
+    - name: Cache - Save ccache
+      if: always()
+      uses: actions/cache/save@v4
+      with:
+        path: ~/.ccache
+        key: ${{ inputs.cache-primary-key }}

.github/actions/setup-ccache/action.yml

@@ -0,0 +1,56 @@
+name: Setup ccache
+description: Restore ccache from cache and configure ccache.conf
+
+inputs:
+  cache-key-prefix:
+    description: Cache key prefix (e.g. ccache-sitl)
+    required: true
+  max-size:
+    description: Max ccache size (e.g. 300M)
+    required: false
+    default: '300M'
+  base-dir:
+    description: ccache base_dir value
+    required: false
+    default: '${GITHUB_WORKSPACE}'
+  install-ccache:
+    description: Install ccache via apt before configuring
+    required: false
+    default: 'false'
+
+outputs:
+  cache-primary-key:
+    description: Primary cache key (pass to save-ccache)
+    value: ${{ steps.restore.outputs.cache-primary-key }}
+
+runs:
+  using: composite
+  steps:
+    - name: Cache - Install ccache
+      if: inputs.install-ccache == 'true'
+      shell: bash
+      run: apt-get update && apt-get install -y ccache
+
+    - name: Cache - Restore ccache
+      id: restore
+      uses: actions/cache/restore@v4
+      with:
+        path: ~/.ccache
+        key: ${{ inputs.cache-key-prefix }}-${{ github.ref_name }}-${{ github.sha }}
+        restore-keys: |
+          ${{ inputs.cache-key-prefix }}-${{ github.ref_name }}-
+          ${{ inputs.cache-key-prefix }}-${{ github.base_ref || 'main' }}-
+          ${{ inputs.cache-key-prefix }}-
+
+    - name: Cache - Configure ccache
+      shell: bash
+      run: |
+        mkdir -p ~/.ccache
+        echo "base_dir = ${{ inputs.base-dir }}" > ~/.ccache/ccache.conf
+        echo "compression = true" >> ~/.ccache/ccache.conf
+        echo "compression_level = 6" >> ~/.ccache/ccache.conf
+        echo "max_size = ${{ inputs.max-size }}" >> ~/.ccache/ccache.conf
+        echo "hash_dir = false" >> ~/.ccache/ccache.conf
+        echo "compiler_check = content" >> ~/.ccache/ccache.conf
+        ccache -s
+        ccache -z