ci: add debug and FFI fuzz guards

[membphis]

Summary

• Add a debug scalar Rust CI gate with cargo test --no-default-features.
• Add fuzz_ffi_ops to the short PR fuzz guard.
• Document the local fuzz_ffi_ops fuzz command in CONTRIBUTING.

Closes #101.
Closes #102.

Test Plan

• git diff --check -- .github/workflows/ci.yml CONTRIBUTING.md
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/ci.yml"); puts "YAML parsed"'
• cargo test --no-default-features
• cargo +nightly fuzz run fuzz_ffi_ops -- -runs=1

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 16768ef9-fa0b-4795-a673-53fd4188f545

📥 Commits

Reviewing files that changed from the base of the PR and between 3cc1d32 and 746f521.

📒 Files selected for processing (2)

• .github/workflows/ci.yml
• CONTRIBUTING.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/issues-101-102-ci-guards

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