Show observed slot size in GPT diagnostics

[ChristianPavilonis]

Summary

• retain GPT-reported slotRenderEnded.size separately from an observed outer slot box
• measure uniquely bound filled slots after render and update on resize when supported
• display and export both metrics, with stale-cycle guards and SafeFrame limitations documented

Closes #1031

Validation

• cd crates/trusted-server-js/lib && npx vitest run (45 files, 834 tests)
• cd crates/trusted-server-js/lib && npx vitest run test/integrations/gpt_diagnostics/slot_size_observer.test.ts (4 tests)
• cd crates/trusted-server-js/lib && npm run format
• git diff --check