Fix large piped output hanging and duplicating bytes

[kklimuk]

Summary

Fixes #8.

Large piped output can hang and repeat its first 64 KiB after a dependency accesses process.stdout. Use lazy Bun FileSinks for stdout and stderr, awaiting each write and flush before the CLI exits. Text/JSON output and the in-process capture API stay compatible.

Add bounded real-pipe regressions for backpressure, UTF-8 byte integrity, sequential writes, immediate exit, restored capture, and early consumer closure. Exercise full CLI Markdown/AST output and run the output contract on macOS with Bun 1.3.10 and latest. Update the output guidance and CI documentation.

Test plan

• bun run check (lint, types, dead code).
• 87 output-contract tests pass on macOS ARM64 with Bun 1.3.10 and installed Bun 1.4.1-canary.1.
• Source, npm bundle, and standalone executable: large Markdown, AST, and raw XML output matches file redirection byte-for-byte through real pipes; help/version, stderr, errors, mutation acknowledgments, and terminal output checked.
• Bun 1.3.10-built bundle and standalone executable pass large-output and document-operation smoke checks.
• Adversarial security, correctness, and product review; findings addressed.

No LibreOffice checks were run during final review. Native Windows/Linux execution and the new GitHub CI jobs have not been run locally. The pre-commit full-suite hook was skipped to avoid launching LibreOffice; the checks above were run separately.