fix: router injection guard was matching __stxRouterConfig as false positive

The guard `template.includes('__stxRouter')` was matching the layout's
`window.__stxRouterConfig` variable, thinking the router was already
injected. Changed to check for the router's actual guard signature
(`__stxRouter)return` or `__stxRouter=true`) instead of a substring.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: glennmichael123

packages/stx/src/process.ts

@@ -672,8 +672,8 @@ export function injectRouterScript(template: string): string {
     return template
   }
 
-  // Don't inject if already present
-  if (template.includes('__stxRouter')) {
+  // Don't inject if already present (check for the router's own guard, not config references)
+  if (template.includes('__stxRouter)return') || template.includes('__stxRouter=true')) {
     return template
   }