feat: introduce benchmarking

.github/workflows/benchmark.yml

@@ -0,0 +1,83 @@
+name: Benchmark PR
+
+on:
+  pull_request:
+    types: [opened, synchronize, reopened]
+
+permissions:
+  contents: read
+  pull-requests: write
+
+jobs:
+  benchmark:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+        with:
+          fetch-depth: 0
+
+      - uses: dtolnay/rust-toolchain@stable
+
+      - uses: actions/cache@v5
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            target
+          key: ${{ runner.os }}-bench-${{ hashFiles('**/Cargo.lock') }}
+
+      - name: Benchmark main branch
+        run: |
+          git fetch origin ${{ github.base_ref }}
+          git checkout origin/${{ github.base_ref }}
+          cargo bench --bench flatten -- --save-baseline main
+
+      - name: Benchmark PR branch
+        run: |
+          git checkout ${{ github.event.pull_request.head.sha }}
+          cargo bench --bench flatten -- --baseline main > bench_output.txt 2>&1
+          cat bench_output.txt
+
+      - name: Comment PR
+        uses: actions/github-script@v9
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          script: |
+            const fs = require('fs');
+            const output = fs.readFileSync('bench_output.txt', 'utf8');
+
+            const body = `## Benchmark Results
+
+            \`\`\`
+            ${output}
+            \`\`\`
+
+            ---
+            Benchmarked on commit \`${{ github.event.pull_request.head.sha }}\``;
+
+            const { data: comments } = await github.rest.issues.listComments({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              issue_number: context.issue.number,
+            });
+
+            const botComment = comments.find(c =>
+              c.user.login === 'github-actions[bot]' &&
+              c.body.includes('Benchmark Results')
+            );
+
+            if (botComment) {
+              await github.rest.issues.updateComment({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                comment_id: botComment.id,
+                body: body
+              });
+            } else {
+              await github.rest.issues.createComment({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                issue_number: context.issue.number,
+                body: body
+              });
+            }