fix: stdout/stderr interleaving in build output (issue #2511)

[wolfv]

Summary

This PR fixes a critical issue where stdout and stderr from build processes were appearing in random order in the output logs. The problem was caused by reading stdout and stderr as separate streams and racing them with tokio::select!, which couldn't recover the true ordering due to independent buffering.

Key Changes

• Merged stdout and stderr into a single OS pipe: Both streams now write to the same pipe, preserving the kernel's ordering of writes
• Simplified output handling: Removed the dual-stream reading logic with tokio::select! in favor of reading from a single merged pipe
• Cross-platform pipe handling: Added platform-specific code to convert OS pipe handles to std::fs::File for both Unix and Windows
• Updated output structure: Changed from separate stdout_log and stderr_log to a single output_log, with stderr now empty (all output merged into stdout)
• Added regression test: Implemented test_stdout_stderr_interleaved_in_write_order() that runs 20 iterations to verify consistent ordering

Implementation Details

• Uses the os_pipe crate to create OS-level pipes that preserve write ordering
• Drops the parent's write end of the pipe before reading to ensure EOF is properly detected
• Wraps the pipe's read end as a tokio::fs::File to perform async reads on the blocking thread pool
• The merged pipe approach is equivalent to the documented exec 2>&1 workaround but applied uniformly regardless of the shell interpreter
• All output filtering and log file writing logic remains unchanged, just operating on the merged stream

https://claude.ai/code/session_01TTZZd4NWFdMcEnGyWwyefx