Add opt-in GPT runtime diagnostics overlay #973

User story

As a publisher operator, I want an opt-in, tab-local GPT runtime diagnostics overlay so that I can inspect GPT-observed slot lifecycles and exact DOM bindings without modifying publisher or auction behavior.

Scope

• Deployment-controlled and query-activated diagnostics runtime.
• Bounded GPT lifecycle capture with conservative correlation.
• Read-only API, local export, closed-shadow panel, and viewport badges.
• Automated browser, runtime, and privacy/non-interference coverage.