Clarify GPT auction diagnostics evidence

[ChristianPavilonis]

Summary

• classify GPT requests only from observed auction evidence and keep server and browser timing clocks separate
• distinguish auction winners, Prebid candidates, delivery evidence, and confirmed served creatives
• add accessible Ad #N · Request #M navigation between page badges and request panels
• correlate Prebid diagnostics with Prebid's auction IDs without changing behavior when diagnostics are inactive
• reorganize panel facts and document every visible diagnostics label

This PR is stacked on #1121.

Validation

• cargo test-fastly
• cargo test-axum
• cargo test-cloudflare
• cargo test-spin
• parity integration tests
• all configured Clippy targets
• cargo fmt --all -- --check
• JavaScript Vitest suite, 904 tests
• JavaScript ESLint, Prettier, and bundle build
• documentation formatting, lint, and VitePress build
• Playwright discovered all 3 GPT diagnostics browser tests

The Playwright browser tests could not execute locally because Docker access was denied at /var/run/docker.sock.

Closes #1081