Move CodSpeed library setup to Python script

Extract inline shell from CI workflow into script/setup_codspeed_lib.py.
Pins codspeed-cpp to a specific commit SHA for reproducibility.

.github/workflows/ci.yml

@@ -327,37 +327,12 @@ jobs:
           python-version: ${{ env.DEFAULT_PYTHON }}
           cache-key: ${{ needs.common.outputs.cache-key }}
 
-      - name: Set up CodSpeed benchmark library
-        # CodSpeed requires their google_benchmark fork for instrumentation.
-        # Clone and create a PlatformIO-compatible library layout.
-        run: |
-          CODSPEED_SHA=d6b4111428ae1f1667fec9bff009522378d5d347
-          git clone https://github.com/CodSpeedHQ/codspeed-cpp.git /tmp/codspeed-cpp
-          git -C /tmp/codspeed-cpp checkout "$CODSPEED_SHA"
-          # Create library.json combining google_benchmark + codspeed core
-          cat > /tmp/codspeed-cpp/google_benchmark/library.json << 'LIBJSON'
-          {
-            "name": "benchmark",
-            "version": "0.0.0",
-            "build": {
-              "flags": [
-                "-I/tmp/codspeed-cpp/core/include",
-                "-I/tmp/codspeed-cpp/core/instrument-hooks",
-                "-DHAVE_STD_REGEX",
-                "-DHAVE_STEADY_CLOCK",
-                "-DBENCHMARK_STATIC_DEFINE"
-              ],
-              "srcFilter": ["+<src/*.cc>", "+</tmp/codspeed-cpp/core/src/*.cpp>"],
-              "includeDir": "include"
-            }
-          }
-          LIBJSON
-
       - name: Build benchmarks
         id: build
         run: |
           . venv/bin/activate
-          BENCHMARK_LIB="symlink:///tmp/codspeed-cpp/google_benchmark" \
+          BENCHMARK_LIB=$(python script/setup_codspeed_lib.py)
+          BENCHMARK_LIB="$BENCHMARK_LIB" \
             script/cpp_benchmark.py --all --build-only 2>&1 | tee /tmp/bench-build.log
           BINARY=$(tail -1 /tmp/bench-build.log)
           echo "binary=$BINARY" >> $GITHUB_OUTPUT