feat: bind snapshots and refs to exact targets

[gcharang]

Summary

• bind snapshots to the attached tab and fragment-complete main-frame URL
• scope accessibility refs to the extension session, attachment, and generation
• serialize command admission and isolate replacement WebSocket sessions
• enforce one authoritative backend extension connection
• add the protocol 0.6.0 and connector 0.4.0 release changeset

Verification

• Node 22.23.1 / pnpm 11.5.2
• frozen install
• publishable package build
• recursive typecheck
• 275 tests passed
• recursive build
• changeset resolves protocol 0.6.0 and connector 0.4.0