ci: pin protoc v36.2 with checksums across wheel builds (#4535)

## Summary

The Python wheel builds used different protoc versions: v24.4 on Linux,
v21.12 on Windows, and whatever Homebrew shipped on macOS. None of them
verified the download. A new composite action,
`.github/actions/setup-protoc`, now holds one pin, v36.2, with a SHA-256
for each platform's zip.

- macOS and Windows: the action downloads protoc, checks its hash, and
installs it.
- Linux: protoc has to be installed inside the manylinux container, so
the build reads the version and hash from the action's outputs and
checks the hash there.

protobuf does not publish checksum files for protoc. The pinned hashes
are the digests GitHub recorded for the v36.2 release assets. Older
releases such as v24.4 have none, which is why this moves to v36.2.
protoc only runs at build time, through `prost-build` in the Lance
crates. The Linux protoc binary is statically linked, so it runs in the
manylinux 2_17 container.

The action is also added to the path triggers of `pypi-publish.yml` and
`python.yml`, so a change to the pin runs the wheel builds.

Follow-up to
https://github.com/lancedb/lancedb/pull/4522#issuecomment-6160848344.
The sdist job in that PR can use the same action once this merges.

## Evidence

Running the action's install script locally on linux-aarch_64 installs
`libprotoc 36.2`, and a wrong hash fails with `protoc checksum
mismatch`. `cargo check -p lancedb` with `PROTOC` set to protoc 36.2
builds the Lance protos. The PyPI Publish dry run on this PR covers the
Linux, macOS and Windows builds.

## Merge Risk

**Door:** Two-way. CI only.
**Blast Radius:** Python wheel builds.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

.github/actions/setup-protoc/action.yml

@@ -0,0 +1,80 @@
+name: Setup protoc
+description: Install the protoc version pinned for wheel builds, and expose it for builds that install protoc themselves
+
+inputs:
+  install:
+    description: "Install protoc on the runner. Set to false when the build runs in a container (e.g. manylinux) and installs protoc there."
+    required: false
+    default: "true"
+
+outputs:
+  version:
+    description: "The pinned protoc version, e.g. 36.2"
+    value: ${{ steps.pin.outputs.version }}
+  linux-x86_64-sha256:
+    description: "SHA-256 of the protoc linux-x86_64 zip"
+    value: ${{ steps.pin.outputs.linux-x86_64-sha256 }}
+  linux-aarch_64-sha256:
+    description: "SHA-256 of the protoc linux-aarch_64 zip"
+    value: ${{ steps.pin.outputs.linux-aarch_64-sha256 }}
+
+runs:
+  using: composite
+  steps:
+    # protobuf does not publish checksum files for protoc. These are the digests
+    # GitHub records for the release assets; when bumping, copy them from
+    # `gh api repos/protocolbuffers/protobuf/releases/tags/v<version>`.
+    - name: Set protoc pin
+      id: pin
+      shell: bash
+      run: |
+        {
+          echo "version=36.2"
+          echo "linux-x86_64-sha256=121f6c7afe1d4d0e3ea6aab9432038599250134cbf4474cb1167d2c7decd4278"
+          echo "linux-aarch_64-sha256=8b8f18bd2b30346efbc698dd5a73dd7c805f3ef8380f6dfc95c768f3f1852f6a"
+          echo "osx-universal_binary-sha256=83d0dc80672f53486f54299bca6177159f9cad1fd54d4d3143eeea53c4423e1b"
+          echo "win64-sha256=f0c128dc0d8492eceece83bb459a4c0e316764b929ffbf1aa416357fd644edd3"
+        } >> "$GITHUB_OUTPUT"
+    - name: Install protoc
+      if: ${{ inputs.install == 'true' }}
+      shell: bash
+      env:
+        PROTOC_VERSION: ${{ steps.pin.outputs.version }}
+        RUNNER_OS_NAME: ${{ runner.os }}
+        RUNNER_ARCH_NAME: ${{ runner.arch }}
+        LINUX_X86_64_SHA256: ${{ steps.pin.outputs.linux-x86_64-sha256 }}
+        LINUX_AARCH_64_SHA256: ${{ steps.pin.outputs.linux-aarch_64-sha256 }}
+        OSX_SHA256: ${{ steps.pin.outputs.osx-universal_binary-sha256 }}
+        WIN64_SHA256: ${{ steps.pin.outputs.win64-sha256 }}
+      run: |
+        set -euo pipefail
+        case "$RUNNER_OS_NAME/$RUNNER_ARCH_NAME" in
+          Linux/X64) platform="linux-x86_64"; sha256="$LINUX_X86_64_SHA256" ;;
+          Linux/ARM64) platform="linux-aarch_64"; sha256="$LINUX_AARCH_64_SHA256" ;;
+          macOS/*) platform="osx-universal_binary"; sha256="$OSX_SHA256" ;;
+          Windows/X64) platform="win64"; sha256="$WIN64_SHA256" ;;
+          *) echo "Unsupported runner: $RUNNER_OS_NAME/$RUNNER_ARCH_NAME" >&2; exit 1 ;;
+        esac
+        zip="$RUNNER_TEMP/protoc.zip"
+        dest="$RUNNER_TEMP/protoc"
+        curl -fsSL "https://github.com/protocolbuffers/protobuf/releases/download/v${PROTOC_VERSION}/protoc-${PROTOC_VERSION}-${platform}.zip" -o "$zip"
+        # Hash stdin rather than a path: Windows paths contain backslashes,
+        # which make sha256sum escape its output.
+        if [ "$RUNNER_OS_NAME" = "macOS" ]; then
+          actual=$(shasum -a 256 < "$zip" | cut -d' ' -f1)
+        else
+          actual=$(sha256sum < "$zip" | cut -d' ' -f1)
+        fi
+        if [ "$actual" != "$sha256" ]; then
+          echo "protoc checksum mismatch: expected $sha256, got $actual" >&2
+          exit 1
+        fi
+        if [ "$RUNNER_OS_NAME" = "Windows" ]; then
+          # Git Bash on Windows runners does not reliably ship unzip.
+          7z x -y -o"$dest" "$zip" > /dev/null
+        else
+          unzip -q -o "$zip" -d "$dest"
+        fi
+        rm "$zip"
+        echo "$dest/bin" >> "$GITHUB_PATH"
+        "$dest/bin/protoc" --version