fix: add Windows compatibility for signal handling

[AppleLamps]

Problem

The playground fails to compile on Windows with:

undefined: syscall.SIGTSTP

Windows doesn't have SIGTSTP (suspend signal - Ctrl+Z), causing build failures.

Solution

• Split signal handling into platform-specific files using build tags
• signal_unix.go (+build unix) handles Unix-specific signals including SIGTSTP
• Windows uses the default signal handling (os.Interrupt, SIGTERM)
• Maintains the same functionality on both platforms

Testing

• Tested on Windows 11 with Go 1.21+
• Server starts successfully with go run .\cmd\playground\main.go start
• Graceful shutdown works with Ctrl+C

[CLAassistant]

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