fix: preserve miden wrapper child exit codes

[omerbek]

Summary

Preserves the wrapped component process exit code when miden successfully dispatches to a child command.

Fixes #243.

Why

Config::execute_command() already returns the child process ExitStatus, but miden_wrapper converted every non-zero child status into an anyhow error. Since main returned anyhow::Result<()>, all child failures collapsed to process exit code 1 at the wrapper boundary.

That makes CI callers unable to distinguish component-specific exit codes, usage errors, or panic exits from wrapped tools.

Changes

• Return std::process::ExitCode through the top-level command execution path.
• Convert normal child exit statuses to the same wrapper exit code instead of turning them into wrapper errors.
• Preserve the current error path for failures to resolve or spawn the child process.
• Map Unix signal termination to 128 + signal where available, with a failure fallback otherwise.
• Add a Unix regression test that installs a fixture miden-vm exiting 42 and verifies miden vm exits with 42.

Testing

• cargo check
• cargo clippy --lib --bins -- -D warnings
• cargo test --test miden integration_miden_preserves_component_exit_code (Windows: test is cfg(unix), so it compiles and is filtered out locally; it exercises the shell-script fixture on Unix CI)
• cargo test --test operations --no-run
• git diff --check

Notes from local Windows validation:

• cargo test / cargo clippy --all-targets --all-features currently compile unrelated Unix-only test code paths on Windows (std::os::unix::fs::symlink in existing tests), so they do not complete locally.
• cargo fmt --check reports repository-wide newline-style issues under this Windows checkout; the submitted diff was checked for whitespace with git diff --check and the PR compare was verified to avoid CRLF-only churn.

[bitwalker]

Your commits need to be signed

[omerbek]

The commit is now SSH-signed and GitHub shows it as verified (a4f9ad2).

The remaining CI failure is separate from the signature check: verify / release lint and verify / package closure both fail at cargo make release ... with Task "release" not found. I tried updating the branch onto next, but GitHub rejected the branch update because the base history includes a workflow file change and my token cannot update .github/workflows/ci.yml. The code checks that ran against this head are green (build, lint, unit tests, integration tests, rustfmt).