add benchmark

.github/workflows/bench-build.yaml

@@ -0,0 +1,41 @@
+name: Benchmark build
+
+# Compile `chia-pos2` Criterion benchmarks only (`--no-run`). Does not execute them.
+
+on:
+  push:
+    branches:
+      - main
+    tags:
+      - "**"
+  pull_request:
+    branches:
+      - "**"
+
+concurrency:
+  group: ${{ github.event_name == 'pull_request' && format('{0}-{1}', github.workflow_ref, github.event.pull_request.number) || github.run_id }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  compile-benches:
+    name: Compile benches (${{ matrix.os }})
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os:
+          - ubuntu-24.04
+          - macos-14
+          - windows-2022
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v5
+
+      - name: Set up Rust
+        uses: dtolnay/rust-toolchain@1.85.0
+
+      - name: Build benchmark targets (no run)
+        run: cargo bench -p chia-pos2 --no-run