feat: add configurable stdout observer destinations

[Krastanov]

StdoutObserver currently captures VM text only in memory. Add buffered(), stdout(), and file(File) constructors so callers can select memory capture, process stdout, or disk output using the same concrete observer type. Default and output() -> &[u8] remain compatible with existing callers; stdout/file destinations retain no extra memory copy and return an empty slice from output().

The caller controls file creation, truncation, and append options. Direct writes and observed effects use the selected destination, preserve UTF-8 bytes, and propagate I/O errors. Add explicit flush(), API and guide documentation, and integration tests using real files and a child process to check stdout isolation. tempfile is a test-only dependency.

This is a separate vihaco change related to QuEraComputing/stellarscope#134; it does not change Corvus or its JSON format.

Validation on macOS with Rust 1.96.0:

• Passed cargo test -p vihaco-stdlib (5 unit tests and 6 integration tests).
• Passed cargo test --workspace --doc.
• Passed cargo clippy --workspace --all-targets -- -D warnings.
• Passed cargo fmt --all -- --check and hawkeye check.
• Passed cargo test --workspace --all-targets.
• Independent implementation, documentation, and test review found no remaining issues.

[github-actions]

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[kaihsin]

Can we instead of having multiple output enum type, just redirect std::io::Cursor to what you want to output? I think Cursor is flexible enough, just need to redirect that Cursor buffer to either stdout or file.

[kaihsin]

see above