fix: resolve critical bugs and refactor code structure

P0 Fixes:
- Fix potential panic in factory.go due to unsafe type assertion
- Fix VIP CIDR mask being lost during parsing (was hardcoded to /32)

P1 Fixes:
- Fix go.mod incorrect indirect dependency markers
- Fix receiveLoop blocking issue preventing graceful shutdown

Refactoring:
- Split state.go into state.go, timer.go, priority.go, history.go
- Split monitor.go into monitor.go and manager.go
- Add IncreasePriority() method for complete priority adjustment
- Fix go vet format string warning in test.go

🤖 Generated with [Qoder][https://qoder.com]

internal/vrrp/timer.go

@@ -0,0 +1,64 @@
+package vrrp
+
+import (
+	"sync"
+	"time"
+)
+
+// Timer provides a thread-safe timer with callback support.
+type Timer struct {
+	duration time.Duration
+	timer    *time.Timer
+	callback func()
+	mu       sync.Mutex
+}
+
+// NewTimer creates a new Timer with the specified duration and callback.
+func NewTimer(duration time.Duration, callback func()) *Timer {
+	return &Timer{
+		duration: duration,
+		callback: callback,
+	}
+}
+
+// Start starts or restarts the timer.
+func (t *Timer) Start() {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+
+	if t.timer != nil {
+		t.timer.Stop()
+	}
+
+	t.timer = time.AfterFunc(t.duration, t.callback)
+}
+
+// Stop stops the timer if it's running.
+func (t *Timer) Stop() {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+
+	if t.timer != nil {
+		t.timer.Stop()
+		t.timer = nil
+	}
+}
+
+// Reset stops the current timer and starts a new one with the same duration.
+func (t *Timer) Reset() {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+
+	if t.timer != nil {
+		t.timer.Stop()
+	}
+
+	t.timer = time.AfterFunc(t.duration, t.callback)
+}
+
+// SetDuration updates the timer's duration for future starts.
+func (t *Timer) SetDuration(duration time.Duration) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	t.duration = duration
+}