Add backend regression benchmark harness

Add known-good backend regression benchmark harness

Introduce an ignored integration test that measures backend-sensitive
compress/decompress cases against a known-good commit.

- the test benchmarks the current checkout in release mode
- it creates a detached worktree at KNOWN_GOOD_COMMIT on the same runner
- it generates a tiny temporary Cargo project whose manifest points
  `flate2` at that worktree
- it copies `tests/support/backend-regression-driver.rs` into that
  project and runs it to produce baseline CSV data
- it compares the current results against those baseline measurements and
  fails when the observed slowdown exceeds the combined measurement
  uncertainty of the current run and the baseline run

- run a single backend locally with:
  `cargo test --release --test backend-regression-bench -- --ignored --exact backend_regression_bench --nocapture`
- switch backends the same way the rest of the test matrix does, for
  example:
  `cargo test --release --test backend-regression-bench --features zlib-rs --no-default-features -- --ignored --exact backend_regression_bench --nocapture`
- optionally, inspect `target/backend-bench/*.csv` for the raw baseline/current
  measurements after a run

This is intended to be maintained as a regression guard:
- update KNOWN_GOOD_COMMIT only when intentionally accepting a new
  performance baseline
- keep the driver source reusable and branch-independent so the baseline
  setup stays lightweight and easy to reason about
```

> Add this benchmark (#544 (comment)) to version of the library before #502 was merged to get a baseline for all backends. Store that baseline, and add a CI job that checks against the baseline.
  Consider other uses of performance-critical low-level methods and see if it makes sense to add more kinds of tests to the baseline the most recent version.
  Success means that the baseline as run on main reproduces issue #544 as well, there is no need to attempt to fix it.

Co-authored-by: Sebastian Thiel <sebastian.thiel@icloud.com>

Author: codex

.github/workflows/backend-regression.yml

@@ -0,0 +1,42 @@
+name: Backend performance regression
+
+on:
+  push:
+  pull_request:
+  workflow_dispatch:
+
+jobs:
+  backend-regression:
+    name: Backend regression (${{ matrix.backend.name }})
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    strategy:
+      fail-fast: false
+      matrix:
+        backend:
+          - name: rust_backend
+            command: cargo test --release --test backend-regression-bench -- --ignored --nocapture
+          - name: zlib-rs
+            command: cargo test --release --test backend-regression-bench --features zlib-rs --no-default-features -- --ignored --nocapture
+          - name: zlib
+            command: cargo test --release --test backend-regression-bench --features zlib --no-default-features -- --ignored --nocapture
+          - name: zlib-ng
+            command: cargo test --release --test backend-regression-bench --features zlib-ng --no-default-features -- --ignored --nocapture
+          - name: zlib-ng-compat
+            command: cargo test --release --test backend-regression-bench --features zlib-ng-compat --no-default-features -- --ignored --nocapture
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      - name: Install Rust
+        run: rustup update stable --no-self-update && rustup default stable
+        shell: bash
+      - name: Run backend regression benchmark
+        run: ${{ matrix.backend.command }}
+      - name: Upload backend benchmark CSV
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: backend-regression-${{ matrix.backend.name }}
+          path: target/backend-bench/${{ matrix.backend.name }}*.csv