fix: prevent 'closed pipe' errors with thread-safe segment writer

[javi11]

Summary

Fixes the "bufpipe: read/write on closed pipe" error that occurs when background download goroutines attempt to write to segment pipes that have been closed due to reader cancellation or context cancellation.

Problem

Race condition in internal/usenet/segment.go:

1. Download goroutine calls cp.Body() which writes to segment writer
2. Meanwhile, Close() is called (from reader close or context cancellation)
3. segment.Close() closes the writer pipe
4. In-flight Body() write operation fails with "closed pipe" panic

Solution

Implemented thread-safe writer wrapper:

• Added closed flag to track segment state
• Created safeWriter that checks closed status before writes
• Returns io.ErrClosedPipe instead of causing panic
• Made Close() idempotent to handle multiple calls safely

Changes

Modified Files

• internal/usenet/segment.go - Thread-safe writer implementation

New Files

• internal/usenet/segment_test.go - Comprehensive test coverage (8 tests)

Testing

✅ All 16 tests pass (8 new + 8 existing)
✅ No race conditions detected with -race flag
✅ Existing functionality preserved

Test Coverage

• Write after close behavior
• Concurrent write and close race conditions
• Idempotent close operations
• Multiple concurrent writers with close
• Nil writer/segment edge cases
• Race detector validation (20 iterations)

Impact

• No more "closed pipe" panics during downloads
• Graceful error handling when segments close during download
• Downloads can be safely cancelled without crashes
• Error is handled by existing retry logic

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