feat(proxy): report blocked packages on stop, exit 1 if any were blocked

pmg proxy stop now waits for the proxy process to exit, reads the final
blocked count written to the state file on shutdown, and exits non-zero
when one or more packages were blocked during the session.

This gives CI a clear failure signal from the proxy itself, separate
from the package manager's own exit code.

internal/proxystate/state.go

@@ -11,9 +11,10 @@ import (
 const stateFileName = "proxy-state.json"
 
 type State struct {
-	PID        int    `json:"pid"`
-	Addr       string `json:"addr"`
-	CACertPath string `json:"ca_cert_path"`
+	PID          int    `json:"pid"`
+	Addr         string `json:"addr"`
+	CACertPath   string `json:"ca_cert_path"`
+	BlockedCount int    `json:"blocked_count"`
 }
 
 func StatePath(configDir string) string {