Design mobile ad-rendering trace endpoint

[prk-Jr]

Summary

• establish a reviewable design boundary before implementing the mobile ad-rendering trace requested by Create debug endpoint for mobile user to trace ad rendering #1050
• define a mobile-first reproduction and export journey that does not require credentials, developer tools, a target URL, or a server-side report database
• separate privacy-safe server-auction, GPT delivery, and creative-rendering evidence so the report does not claim correlations it cannot prove

Changes

File	Change
docs/superpowers/specs/2026-09-01-mobile-ad-render-trace-endpoint-design.md	Defines UX, routing, configuration, schemas, live auction transport, opaque slot correlation, privacy, failure handling, testing, rollout, acceptance criteria, and implementation sequencing.

Closes

Closes #1108

Implementation remains tracked by #1050. Related observability and timing follow-ups remain tracked by #1081 and #1076.

Test plan

• cargo test-fastly && cargo test-axum — not run; documentation-only change
• cargo clippy-fastly && cargo clippy-axum — not required for the documentation-only change
• cargo fmt --all -- --check
• JS tests: cd crates/trusted-server-js/lib && npx vitest run — 45 files and 893 tests passed under pinned Node 24.12.0
• JS format: cd crates/trusted-server-js/lib && npm run format
• Docs format: cd docs && npm run format
• WASM build: cargo build --package trusted-server-adapter-fastly --release --target wasm32-wasip1 — not run; documentation-only change
• Manual testing via fastly compute serve — not applicable
• Other: independent adversarial design review approved; git diff --check main...HEAD passed

Checklist

• Changes follow CLAUDE.md conventions
• No unwrap() in production code — no production code changed
• Uses tracing macros (not println!) — no logging code changed
• New code has tests — no code added; the spec defines the required implementation test strategy
• No secrets or credentials committed