Implement direct Tinybird auction telemetry ingestion

[ChristianPavilonis]

Summary

Implement direct, best-effort Tinybird Events API ingestion from Fastly Compute for auction telemetry.

Scope

• Add privacy-preserving auction telemetry row construction in core.
• Wire a Fastly fire-and-forget Tinybird sink using Secret Store APPEND tokens.
• Instrument /auction, /__ts/page-bids, and initial-navigation SSAT lifecycle paths.
• Add Tinybird datasources, rollups, endpoint pipes, fixtures, and test contracts.

Acceptance criteria

• Telemetry failures never change customer responses.
• Tinybird ingestion uses direct Events API POST with send_async and does not await responses.
• No EC IDs, request IDs, IPs, raw UAs, full URLs, query strings, or fragments are emitted.
• Rust, docs, and Tinybird validation pass.