Quantize auction transport timeouts to stabilize Fastly backend names

[prk-Jr]

Summary

• Fastly dynamic backend names deliberately embed the first-byte and between-bytes timeouts so a registration can never be silently reused with a different transport configuration. But deriving those timeouts from the remaining wall-clock auction budget minted a new backend name on nearly every request — defeating cross-request TCP/TLS connection reuse to bidder hosts (Fastly pools connections per backend name) and accumulating registrations toward the per-service dynamic backend limit.
• Compute the effective transport timeout in three regimes: the provider's configured timeout verbatim when the budget allows (a constant, already name-stable), the remaining budget floored to 250ms buckets when the budget is the binding constraint, and exact passthrough for sub-quantum remainders so publishers with sub-250ms configured budgets keep launching. The value feeds both the backend name and the registered configuration, so they cannot diverge; rounding down never extends a transport cap past the auction deadline, which the mediator and dispatched-collect paths rely on to bound the </body> hold.
• Pin predict_name == ensure on the Fastly platform backend with a Viceroy test, since the orchestrator maps SSP responses back to providers by predicted backend name.

Changes

File	Change
crates/trusted-server-core/src/auction/orchestrator.rs	Add TRANSPORT_TIMEOUT_QUANTUM_MS, quantize_transport_timeout_ms, and effective_transport_timeout_ms; apply at the four provider-launch sites (parallel, dispatch, synchronous mediator, collect-path mediator); extend the test stub with an optional timeout recorder and add quantization, sub-quantum-passthrough, and mediator-budget tests
crates/trusted-server-adapter-fastly/src/platform.rs	Add predict_name_matches_ensured_backend_name test pinning the predicted name to the registered name for an identical spec

Closes

Closes #847

Test plan

• cargo test-fastly && cargo test-axum
• cargo clippy-fastly && cargo clippy-axum
• cargo fmt --all -- --check
• JS tests: cd crates/trusted-server-js/lib && npx vitest run
• 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
• Manual testing via fastly compute serve
• Other: cargo test-cloudflare && cargo test-spin, cargo clippy-cloudflare && cargo clippy-cloudflare-wasm && cargo clippy-spin-native && cargo clippy-spin-wasm, and the integration parity suite (cargo test --manifest-path crates/trusted-server-integration-tests/Cargo.toml --test parity) all pass

Checklist

• Changes follow CLAUDE.md conventions
• No unwrap() in production code — use expect("should ...")
• Uses tracing macros (not println!)
• New code has tests
• No secrets or credentials committed

[ChristianPavilonis]

Automated review:

Review Summary

Reviewed PR #865 against main, focusing on the timeout quantization logic in crates/trusted-server-core/src/auction/orchestrator.rs and the Fastly backend-name consistency test in crates/trusted-server-adapter-fastly/src/platform.rs. I did not find blocking correctness, security, data-loss, authorization, or severe compatibility issues.

Findings

P0 / Blockers

None.

P1 / High

None.

P2 / Medium

None.

P3 / Low

None.

CI / Existing Reviews

gh pr checks 865 reports all checks passing, including Rust, JS/TS, formatting, integration, browser, CodeQL, and cross-adapter parity jobs. Existing review/comment queries did not show prior review feedback to duplicate.