fix: harden pause flow with connection isolation and UFFD event handling

Restructure pause to: block new operations (StatusPausing), drain proxy
connections with 5s grace, force-close remaining via context cancellation,
drop page cache, inflate balloon, then freeze vCPUs. Previously connections
could arrive during the pause window and API operations weren't blocked.

Handle UFFD_EVENT_REMOVE/UNMAP/REMAP/FORK gracefully instead of crashing
the UFFD server. These events fire during balloon deflation on snapshot
restore, killing the page fault handler and preventing VM boot.

Also adds ConnTracker.ForceClose() with cancellable context propagated
through the proxy handler, so lingering proxy connections are actively
terminated rather than left dangling.

internal/sandbox/conntracker.go

@@ -1,6 +1,7 @@
 package sandbox
 
 import (
+	"context"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -17,6 +18,20 @@ type ConnTracker struct {
 	// goroutine to exit, preventing goroutine leaks on repeated pause failures.
 	cancelMu    sync.Mutex
 	cancelDrain chan struct{}
+
+	// ctx is cancelled by ForceClose to abort all in-flight proxy requests.
+	// Initialized lazily on first Acquire; replaced by Reset after a failed
+	// pause so new connections get a fresh, non-cancelled context.
+	ctxMu  sync.Mutex
+	ctx    context.Context
+	cancel context.CancelFunc
+}
+
+// ensureCtx lazily initializes the cancellable context.
+func (t *ConnTracker) ensureCtx() {
+	if t.ctx == nil {
+		t.ctx, t.cancel = context.WithCancel(context.Background())
+	}
 }
 
 // Acquire registers one in-flight connection. Returns false if the tracker
@@ -35,6 +50,16 @@ func (t *ConnTracker) Acquire() bool {
 	return true
 }
 
+// Context returns a context that is cancelled when ForceClose is called.
+// Proxy handlers should derive their request context from this so that
+// force-close during pause aborts in-flight proxied requests.
+func (t *ConnTracker) Context() context.Context {
+	t.ctxMu.Lock()
+	defer t.ctxMu.Unlock()
+	t.ensureCtx()
+	return t.ctx
+}
+
 // Release marks one connection as complete. Must be called exactly once
 // per successful Acquire.
 func (t *ConnTracker) Release() {
@@ -65,9 +90,33 @@ func (t *ConnTracker) Drain(timeout time.Duration) {
 	}
 }
 
+// ForceClose cancels all in-flight proxy connections by cancelling the
+// shared context. Connections whose request context derives from Context()
+// will see their requests aborted, causing the proxy handler to return
+// and call Release(). Waits briefly for connections to actually release.
+func (t *ConnTracker) ForceClose() {
+	t.ctxMu.Lock()
+	if t.cancel != nil {
+		t.cancel()
+	}
+	t.ctxMu.Unlock()
+
+	// Wait briefly for force-closed connections to call Release().
+	done := make(chan struct{})
+	go func() {
+		t.wg.Wait()
+		close(done)
+	}()
+	select {
+	case <-done:
+	case <-time.After(2 * time.Second):
+	}
+}
+
 // Reset re-enables the tracker after a failed drain. This allows the
 // sandbox to accept proxy connections again if the pause operation fails
-// and the VM is resumed. It also cancels any lingering Drain goroutine.
+// and the VM is resumed. It also cancels any lingering Drain goroutine
+// and creates a fresh context for new connections.
 func (t *ConnTracker) Reset() {
 	t.cancelMu.Lock()
 	if t.cancelDrain != nil {
@@ -81,5 +130,10 @@ func (t *ConnTracker) Reset() {
 	}
 	t.cancelMu.Unlock()
 
+	// Replace the cancelled context with a fresh one.
+	t.ctxMu.Lock()
+	t.ctx, t.cancel = context.WithCancel(context.Background())
+	t.ctxMu.Unlock()
+
 	t.draining.Store(false)
 }