optimize command execution by reducing redundant clones

[google-labs-jules]

This PR optimizes the src/bootstrap tool by reducing unnecessary data cloning and redundant calculations.

💡 What:

1. Shared Output Storage: Changed CommandOutput to store stdout and stderr as Option<Arc<[u8]>> instead of Option<Vec<u8>>.
2. Avoid Redundant Fingerprinting: Updated finish_process to accept a pre-calculated CommandFingerprint instead of recalculating it from the BootstrapCommand.

🎯 Why:

• Performance: CommandOutput is frequently cloned when retrieved from the CommandCache or during step execution. With Vec<u8>, this was an $O(N)$ operation. With Arc<[u8]>, it's $O(1)$.
• Efficiency: command.fingerprint() is a relatively expensive operation that clones all command arguments and environment variables. Calling it again in finish_process was redundant since the fingerprint was already available in wait_for_output.

📊 Impact:

• Reduces data cloning on every cache hit. For commands with large outputs, this significantly reduces memory pressure and CPU time.
• Reduces redundant allocations on the failure/verbose reporting path.

🔬 Measurement:

• Verified with python3 x.py check src/bootstrap and python3 x.py test src/bootstrap.
• Internal tests for command execution pass as expected.

PR created automatically by by @lang-tempo

[google-labs-jules]

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