docs(dx): close cookbook and migration path

[ExplodingUFO]

Summary

• make the public onboarding path linear from Starter -> HelloWorld -> HelloWorld.Avalonia -> ConsumerSample -> Demo, with HostSample explicitly positioned as proof-only
• align migration/support pages so retained MVVM is clearly a compatibility bridge, not a primary route
• add the missing Starter.Avalonia README and update status pages to reflect the shipped runtime inspection, timeline, and perf overlay surfaces

Verification

• git diff --check
• rg checks confirming the cookbook ladder and retained-bridge wording across the updated docs