release: publish standalone bwrap artifacts (#21256)

**Summary**
- Build Linux `bwrap` before the main release binaries.
- Export the release `bwrap` SHA-256 as `CODEX_BWRAP_SHA256` so the
Codex binary can verify the bundled fallback.
- Sign, stage, and upload `bwrap` alongside the primary Linux release
artifacts.

**Verification**
- YAML parse check for `.github/workflows/rust-release.yml`











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[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/21256).
* #21257
* __->__ #21256

.github/workflows/rust-release.yml

@@ -255,6 +255,24 @@ jobs:
         with:
           target: ${{ matrix.target }}
 
+      - if: ${{ contains(matrix.target, 'linux') }}
+        name: Build bwrap and export digest
+        shell: bash
+        run: |
+          set -euo pipefail
+          target="${{ matrix.target }}"
+          cargo build --target "$target" --release --timings --bin bwrap
+
+          bwrap_path="target/${target}/release/bwrap"
+          if [[ ! -f "$bwrap_path" ]]; then
+            echo "bwrap binary ${bwrap_path} not found"
+            exit 1
+          fi
+
+          digest="$(sha256sum "$bwrap_path" | awk '{print $1}')"
+          echo "CODEX_BWRAP_SHA256=${digest}" >> "$GITHUB_ENV"
+          echo "Built bwrap ${bwrap_path} with sha256:${digest}"
+
       - name: Cargo build
         shell: bash
         run: |
@@ -278,7 +296,7 @@ jobs:
         with:
           target: ${{ matrix.target }}
           artifacts-dir: ${{ github.workspace }}/codex-rs/target/${{ matrix.target }}/release
-          binaries: ${{ matrix.binaries }}
+          binaries: ${{ matrix.binaries }} bwrap
 
       - if: ${{ runner.os == 'macOS' }}
         name: MacOS code signing (binaries)
@@ -361,6 +379,12 @@ jobs:
             fi
           done
 
+          if [[ "${{ matrix.target }}" == *linux* && "${{ matrix.bundle }}" == "primary" ]]; then
+            cp "target/${{ matrix.target }}/release/bwrap" "$dest/bwrap-${{ matrix.target }}"
+            cp "target/${{ matrix.target }}/release/bwrap.sigstore" \
+              "$dest/bwrap-${{ matrix.target }}.sigstore"
+          fi
+
           if [[ "${{ matrix.build_dmg }}" == "true" ]]; then
             cp target/${{ matrix.target }}/release/codex-${{ matrix.target }}.dmg "$dest/codex-${{ matrix.target }}.dmg"
           fi