improve main.zig based on established zig project patterns
- add thread_safe = true to GPA (required for multi-threaded usage)
- use detach() instead of misleading defer join() (consumer never returns)
- switch from debug.print to std.log.scoped for proper log levels
- group imports: stdlib → external → internal
patterns verified against ghostty, tigerbeetle, zap, http.zig
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>