Harden resident target bootstrap

[miguelg719]

Summary

• add V3Context connection/disconnection hooks needed by the resident coordinator
• restrict initial and future auto-attachment to supported web targets
• defer broad page instrumentation until stagehand.init
• safely resume and detach ignored paused targets
• coalesce duplicate target attachment attempts and avoid retaining detached OOPIF sessions
• avoid creating an unsolicited blank page during resident bootstrap

Scope

Stack 3/6, based on the browser-proxy resolver PR. This PR owns V3Context bootstrap and target-safety behavior only; it does not start the resident worker lifecycle.

Review focus

The safety boundary is isSupportedWebTarget plus the initial and future attachment paths in understudy/context.ts. Browser, worker, service-worker, background-page, and unrelated extension targets are excluded.

Validation

• resident target-safety and V3Context lifecycle tests
• complete server suite on this branch
• complete TypeScript/package/rule suite at the stack tip

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Latest commit: 5859b7c

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