Add Graph IR canonicalization and spawn rule loader

[JPrier]

Summary

• add canonical GraphIR structures for spawn rules, execution limits, checkpoints, and deterministic serialization/hashing
• capture build manifest metadata via build script and include it in blake3 IR hashes
• provide spawn_rules.json loader that prefers the working directory, normalizes defaults, and validates early

Testing

• cargo clippy --all-targets --all-features -- -D warnings
• cargo test
• ruff check
• mypy src
• pytest

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Codex Task