fix: preserve imports across runtime transitions

[chubes4]

Summary

• safely refresh the same generated companion plugin during overwrite imports while continuing to reject foreign block registrations
• accept media-rich script-heavy source pages and retain inferred Google Fonts fallbacks
• preserve lifecycle handoff fields and assign a server-owned invocation identity
• return a continuation when dependency activation succeeds but initialization requires a fresh request

Related integration: Automattic/studio#3952

Verification

• npm test
• git diff --check
• fresh import with automatic dependency activation, followed by continuation in a second invocation
• overwrite import preserving stable page IDs and canonical content

AI assistance

OpenAI GPT-5.6 Sol through OpenCode investigated the cross-runtime failures, implemented the fixes and contract coverage, and ran verification. Chris Huber reviewed and owns every line.