Replace std::process::exit with ExitCode enum implementing Termination

[Copilot]

The code was calling std::process::exit() directly, preventing proper resource cleanup and using non-idiomatic patterns for setting exit codes.

Changes

Exit code enum with Termination trait

• Created ExitCode enum with #[repr(i32)] and explicit discriminants (0, 1, 2, 3, 4, 5, 6, 70)
• Implemented Termination trait allowing direct return from main
• Implemented From<ExitCode> for std::process::ExitCode

Main function signature

• Changed from fn main() -> Result<()> to fn main() -> Result<ExitCode>
• Removed all std::process::exit() calls throughout codebase
• Eliminated .into() conversions (handled by Termination trait)

Return type updates

• LabOutcome::exit_code() and DiffFilterError::exit_code() now return ExitCode enum
• Consistent imports: use crate::exit_code::ExitCode across all files

Before:

fn main() -> Result<()> {
    // ...
    std::process::exit(lab_outcome.exit_code());
}

After:

fn main() -> Result<ExitCode> {
    // ...
    Ok(lab_outcome.exit_code())
}

Additional improvements

• Added error logging before returning Usage exit code for invalid manifest paths
• Added test coverage for exit codes 5 (FILTER_DIFF_MISMATCH) and 6 (FILTER_DIFF_INVALID)
• Formatted code with cargo fmt

Original prompt

Don't use sys::exit to set the return code. Instead, return an integer (or maybe Result of an integer) from main.

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