Add cross-platform benchmark workflow (manual trigger)

Benchmarks Fat JAR+AOT, Fat JAR, JBang, and Python across
ubuntu, windows, and macos. Results written to job summary.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: brunoborges

.github/workflows/benchmark.yml

@@ -0,0 +1,92 @@
+name: Benchmark Generator
+
+on:
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  actions: write
+
+jobs:
+  benchmark:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v6
+
+      - uses: actions/setup-java@v5
+        with:
+          distribution: 'temurin'
+          java-version: '25'
+
+      - uses: jbangdev/setup-jbang@main
+
+      - name: Setup Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.13'
+
+      - name: Build fat JAR
+        run: jbang export fatjar --force --output html-generators/generate.jar html-generators/generate.java
+
+      - name: Build AOT cache
+        run: java -XX:AOTCacheOutput=html-generators/generate.aot -jar html-generators/generate.jar
+
+      - name: Run benchmark
+        shell: bash
+        run: |
+          JAR="html-generators/generate.jar"
+          AOT="html-generators/generate.aot"
+          RUNS=6
+
+          snippet_count=$(find content -name '*.json' | wc -l | tr -d ' ')
+          java_ver=$(java -version 2>&1 | head -1 | sed 's/.*"\(.*\)".*/\1/')
+          os_name="${{ matrix.os }}"
+
+          # Timing helper using bash TIMEFORMAT
+          measure() {
+            TIMEFORMAT='%R'
+            local t
+            t=$( { time "$@" > /dev/null 2>&1; } 2>&1 )
+            echo "$t"
+          }
+
+          bench() {
+            local label="$1"; shift
+            local times=()
+            for ((i = 1; i <= RUNS; i++)); do
+              t=$(measure "$@")
+              times+=("$t")
+            done
+            local cold="${times[0]}"
+            local sum=0
+            for ((i = 1; i < RUNS; i++)); do
+              sum=$(awk "BEGIN {print $sum + ${times[$i]}}")
+            done
+            local warm
+            warm=$(awk "BEGIN {printf \"%.2f\", $sum / ($RUNS - 1)}")
+            echo "${label}|${cold}|${warm}"
+          }
+
+          echo "Running benchmark on $os_name (Java $java_ver, $snippet_count snippets)..."
+
+          aot_result=$(bench "Fat JAR + AOT" java -XX:AOTCache="$AOT" -jar "$JAR")
+          jar_result=$(bench "Fat JAR" java -jar "$JAR")
+          jbang_result=$(bench "JBang" jbang html-generators/generate.java)
+          python_result=$(bench "Python" python3 html-generators/generate.py)
+
+          # Write to GitHub Actions Job Summary
+          {
+            echo "## Benchmark Results — \`$os_name\`"
+            echo ""
+            echo "Java $java_ver · $snippet_count snippets · $RUNS runs (1 cold + $((RUNS - 1)) warm)"
+            echo ""
+            echo "| Method | Cold Start | Warm Average |"
+            echo "|--------|-----------|-------------|"
+            for result in "$aot_result" "$jar_result" "$jbang_result" "$python_result"; do
+              IFS='|' read -r label cold warm <<< "$result"
+              echo "| **$label** | ${cold}s | ${warm}s |"
+            done
+          } >> "$GITHUB_STEP_SUMMARY"