SPEC Specify mobile ad-rendering trace endpoint #1108

Context

Issue #1050 requests a mobile-friendly debug endpoint for tracing ad rendering, but the endpoint contract, privacy boundary, auction evidence model, and user journey require design agreement before implementation begins.

Scope

• define the /_ts/trace mobile activation, reproduction, report, and export journey
• define privacy-safe network and Trusted Server cookie-health fields
• define separate SSAT/Trusted Server auction, GPT delivery, and creative-rendering evidence
• define exact transport and opaque correlation contracts without a database or server-side report store
• define security, truncation, failure, test, rollout, and implementation-sequencing requirements

Acceptance criteria

• Design spec is reviewed and approved
• Scope ownership between Create debug endpoint for mobile user to trace ad rendering #1050, Improvements to TS_CONSOLE for ad observability #1081, and Add auction timeline offsets spec amendment #1076 is explicit
• Implementation can proceed in independently reviewable phases

This ticket covers design approval only. Implementation remains tracked by #1050 and follow-up implementation PRs.