fix(rebuilder): fix rebuilder for windows and goroutine leaks and process lifecycle bugs

[tatang26]

Summary

• Fix critical goroutine and file-descriptor leaks in the file watcher by threading context.Context through Watch()
• Fix debounce timer blocking after shutdown with context-aware channel sends
• Extract platform-specific process handling (setSysProcAttr, terminateProcess, notifyContext) into sysproc_unix.go / sysproc_windows.go
• Fix process lifecycle: explicit context cancellation, post-terminate wait, empty command guard, error propagation from Serve()
• Fix maxServiceNameLen global accumulation across multiple Serve() calls
• Move pflag.Parse() from goroutine to main() for deterministic parsing

Bugs Fixed

Severity	Issue
Critical	Watcher goroutine never stops — no context/done channel in Watch()
Critical	Debounce timer callback blocks on unbuffered channel after shutdown
Critical	fsnotify file descriptor never closed (consequence of watcher leak)
High	defer cancel() inside for loop accumulates contexts per restart
High	Clean process exit invisible — errCh only received errors, not nil
High	reloadCh data race — channels initialized after watcher started
Medium	pflag.Parse() called inside goroutine
Medium	No wait after terminateProcess — process not fully reaped before restart
Medium	Panic on empty command in Procfile
Medium	maxServiceNameLen accumulates across test runs (-count>1)

Testing

• 16 tests pass with -race -count=3
• Coverage: 87.3%
• All tests go through the public Serve() API