docs: add cargo check workaround for macOS + Homebrew LLVM

[shguddn8591]

What does this PR do?

Added a documentation section in CONTRIBUTING.md to help contributors run cargo check on macOS when using Homebrew's llvm@21.

Homebrew's llvm@21 does not support the -fuse-ld=lld flag, which causes linker errors when running cargo check --workspace directly. This PR documents the RUSTFLAGS="" workaround and clarifies that official build commands (bun bd, bun run build) are unaffected.

How did you verify your code works?

• Confirmed that RUSTFLAGS="" cargo check --workspace --keep-going succeeds on macOS with Homebrew llvm@21.
• Verified that the new documentation section renders correctly in CONTRIBUTING.md.

[claude]

Claude Code Review

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[shguddn8591]

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• CONTRIBUTING.md

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Walkthrough

This PR adds macOS-specific troubleshooting documentation to CONTRIBUTING.md, explaining that Homebrew's llvm@21 does not support the -fuse-ld=lld flag and providing a workaround command for running cargo check successfully.

Changes

macOS Homebrew LLVM Troubleshooting Documentation

Layer / File(s)	Summary
macOS Homebrew LLVM troubleshooting guide CONTRIBUTING.md	New subsection documents that Homebrew's llvm@21 does not support -fuse-ld=lld and provides a workaround command using RUSTFLAGS="" cargo check --workspace --keep-going; also notes that official bun bd and bun run build commands apply this fix automatically.

Possibly related issues

• oven-sh/bun#30870: This documentation directly addresses the macOS linker flag problem (llvm@21 not accepting -fuse-ld=lld) that was reported in this issue.

Possibly related PRs

• oven-sh/bun#30708: Both PRs update CONTRIBUTING.md with Rust iteration guidance around cargo check, with this PR adding the specific macOS llvm@21 and -fuse-ld=lld troubleshooting workaround.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title directly and clearly describes the main change: adding a cargo check workaround documentation for macOS with Homebrew LLVM, matching the primary purpose of the PR.
Description check	✅ Passed	The PR description fully covers both required template sections with clear explanations of what was added and how it was verified, meeting all documentation requirements.
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.

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