Add SAM warm-build fast path

[zackees]

Summary

• add SAM-local warm-build fingerprint metadata, watch collection, and expected artifact helpers
• short-circuit SAM builds when fingerprinted inputs and existing artifacts still match
• persist the SAM fingerprint after successful builds and cover the local artifact/watch contract with unit tests

Verification

• cargo test -p fbuild-build --lib sam::orchestrator::tests -- --nocapture
• cargo check -p fbuild-build --lib

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 6b9d72dd-779e-4bd5-bbda-03dabf461052

📥 Commits

Reviewing files that changed from the base of the PR and between 2a9cd23 and a40bbc9.

📒 Files selected for processing (1)

• crates/fbuild-build/src/sam/orchestrator.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/issue-121-sam-fast-path

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