feat(prefork): Enhance prefork management with WatchMaster, CommandProducer, and Windows support

[ReneWerner87]

We've been working on reducing code duplication between Fiber and fasthttp's prefork package. Right now Fiber maintains its own prefork implementation (~200 lines) that reimplements most of what fasthttp/prefork already does - process spawning, child management, the recovery loop - just to hook into a few lifecycle events (logging when a child spawns, showing a startup message after all workers are up, etc.).

This PR adds a small set of optional callbacks to the Prefork struct that make it possible for consumers like Fiber to plug into the prefork lifecycle without copying the whole thing:

• OnChildSpawn(pid int) error - called when a new child is forked, so the caller can track PIDs or run hooks
• OnMasterReady(childPIDs []int) error - called once after all children are up, useful for startup messages or metrics
• OnChildRecover(pid int) error - called when a crashed child gets replaced, for logging/alerting
• CommandProducer(files []*os.File) (*exec.Cmd, error) - allows overriding how child commands are created, which is useful for testing (injecting dummy commands instead of re-executing the binary)

All fields are optional and nil-safe - existing code doesn't need to change. The zero value works exactly as before.

Additionally:

• listenAsChild() extracted to remove duplicated setup code across the three ListenAndServe* methods
• watchMaster now handles Windows correctly (using FindProcess+Wait instead of Getppid polling, which doesn't detect parent exit on Windows)
• Goroutine closures in the spawn/recovery loop pass cmd and pid as parameters instead of capturing the loop variable

You can see the practical usage in the Fiber PR that builds on this: gofiber/fiber#4210 - it replaces Fiber's entire prefork implementation with ~50 lines that configure these callbacks. The rest is handled by this package.

[Copilot]

Pull request overview

Enhances prefork to reduce duplication in downstream consumers (e.g., Fiber) by exposing lifecycle hooks and customizable child command creation, while also improving child/master monitoring behavior on Windows.

Changes:

• Added optional lifecycle callbacks (OnChildSpawn, OnMasterReady, OnChildRecover) and a CommandProducer hook for custom child command creation.
• Refactored child listener setup into listenAsChild() to deduplicate logic across ListenAndServe* methods.
• Updated master-watch logic to correctly detect master exit on Windows and fixed goroutine loop-variable capture in spawn/recovery.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
prefork/prefork.go	Adds callbacks + CommandProducer, refactors child setup, improves Windows master monitoring, and adjusts spawn/recovery loop behavior.
prefork/prefork_test.go	Adds tests around logger/callback fields and CommandProducer usage.

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