ci: cap cargo build parallelism to fit 6Gi runner (fixes 1.85 OOM)

[27Bslash6]

Fixes the 6-day red CI on main (#25).

Root cause (diagnosed live on the ARC runner)

The 1.85 matrix job was killed (SIGKILL/137) ~10 min into cargo test, in the test step, with no step conclusion — and the runner died before it could upload its log blob, which is why GitHub returned BlobNotFound and the failure was undiagnosable from the UI.

cargo reads the node's CPU count (32 threads), not the pod's cgroup CPU quota, so it spawned ~32 parallel rustc in the 6Gi cachekit-lean runner. The build's memory peak (codegen/link) blew the 6Gi ceiling → cgroup memory pressure → thrash for ~10 min → OOM-kill.

Why only 1.85: stable/beta run clippy first (warming build artifacts), so their cargo test is incremental and lower-peak. 1.85 skips fmt/clippy and does the full cold 32-way compile in one step → hits the ceiling.

Fix + proof

Cap CARGO_BUILD_JOBS: "4" so peak memory fits the 6Gi limit. Result on this branch:

	before	after (capped)
1.85	❌ killed at ~10:00 (137)	✅ 75s
stable / beta / wasm	✅	✅

The build doesn't just survive — it's an order of magnitude faster, because it's no longer thrashing under memory pressure.

Note

A more principled fix is making cargo honor the cgroup CPU quota (derive -j from the request), but the env cap is the pragmatic, proven fix. The same footgun likely affects other cachekit Rust repos on these runners (e.g. cachekit-core).

[coderabbitai]

Walkthrough

This PR introduces a single environment variable to the CI workflow that caps Rust build parallelism at 4 jobs, preventing out-of-memory failures during test compilation on the runner.

Changes

CI Parallelism Configuration

Layer / File(s)	Summary
Cargo build jobs environment variable .github/workflows/ci.yml	Adds CARGO_BUILD_JOBS: "4" to the workflow-level env block alongside existing RUSTUP_HOME and CARGO_HOME environment variables, with comments explaining the rationale for limiting parallelism to prevent OOM kills during cargo test.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The PR title accurately describes the main change: capping cargo build parallelism to address OOM issues on 6Gi runners for Rust 1.85.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ci/diag-185-capture

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