Fix: Make OnStateChange callbacks non-blocking

[JustinDFuller]

Note

This PR was generated with Claude Code (claude.ai/code)

Summary

• Fixes the issue where slow OnStateChange callbacks could block the ticker and all nozzle operations
• Callbacks now execute asynchronously in separate goroutines
• Adds proper context cancellation support

Problem

The OnStateChange callback was executed synchronously while holding the mutex lock. This meant:

1. A slow callback would delay the next ticker iteration
2. All DoBool/DoError operations would be blocked during callback execution
3. The ticker couldn't maintain its configured interval

Solution

• Execute callbacks asynchronously using goroutines
• Add context.Context parameter for proper cancellation signaling
• Add Timestamp field to StateSnapshot to record when changes actually occurred
• Implement panic recovery to prevent callback crashes

Breaking Changes

The callback signature has changed from:

OnStateChange func(StateSnapshot)

to:

OnStateChange func(context.Context, StateSnapshot)

Testing

Added comprehensive tests to verify:

• Slow callbacks don't block the ticker
• Panics in callbacks are recovered gracefully
• Context is properly cancelled on Close()
• Timestamps accurately reflect state change times
• All existing tests continue to pass

Test Results

PASS
ok  	github.com/justindfuller/nozzle	77.289s

🤖 Generated with Claude Code