ci: bound all-features test builds to one job

Main CI on ac5b611fe0 exhausted about 15.4 GiB RAM and 11.2 GiB swap
while compiling test binaries, then the runner shut down with exit 143
before any assertion ran. Match the RC parity limit in both Run tests
and the alternate Shared-process workspace qualification step.

Validation: git diff --check and actionlint -shellcheck= pass. Independent
review caught the alternate consumer and it is included. No runtime,
packaging or web source changes; reuse ac5b611fe0's local npm gate:
wrapper 101, SDK 19, extension host 413 passed/7 skipped, web 772 and
security 7, all zero failures. Exact macOS arm64 RC walkthrough at ac5
passed 11 offline scenarios, zero failed; this does not replace the new
candidate's exact-SHA hosted gates or prove paid provider compatibility.

Co-Authored-By: Codex <[email protected]>

.github/workflows/ci.yml

@@ -993,6 +993,7 @@ jobs:
           if [ -n "$monitor" ]; then kill "$monitor" 2>/dev/null || true; fi
           exit "$status"
         env:
+          CARGO_BUILD_JOBS: '1'
           QA_PTY_DIAGNOSTICS_DIR: ${{ runner.temp }}/pty-failures
           CODEWHALE_EXT_HOST_TESTS: '1'
           CODEWHALE_EXT_HOST_BUN_TESTS: '1'
@@ -1030,6 +1031,7 @@ jobs:
         if: github.event_name == 'workflow_dispatch' && inputs.workspace_test_mode == 'shared-process-twice' && matrix.os == 'ubuntu-latest'
         shell: bash
         env:
+          CARGO_BUILD_JOBS: '1'
           EXPECTED_SHA: ${{ inputs.expected_sha }}
           CODEWHALE_EXT_HOST_TESTS: '1'
           CODEWHALE_EXT_HOST_BUN_TESTS: '1'