context.add_init_script() was called in both setup_context() and
_crawl_web(), causing unbounded script accumulation on shared contexts
under concurrent load. Chromium kills the overloaded context, cascading
"Target page, context or browser has been closed" to all concurrent crawls.
Add flag-based dedup: after injecting navigator_overrider or shadow-DOM
scripts, set _crawl4ai_nav_overrider_injected / _crawl4ai_shadow_dom_injected
on the context. Before injecting, check the flag. This preserves context-level
scope (popups/iframes covered) and the fallback for managed/persistent/CDP
paths where setup_context() runs without crawlerRunConfig.
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