net-http construction surface: record four follow-up refinements (ADR-0034 amendments + Guarded mid-stream release)

[NotAProfDev]

Follow-up to PR #66 (ADR-0034, Slice 0 PR 3). A design review surfaced four refinements to the net-http construction surface. This issue tracks recording them (ADR-0034 append-only amendments + spec update) and the one code change they imply.

Refinements

1. Absent RateLimit<K> directive fails closed — tightens ADR-0031 §3's "absent directive defaults to Global", which was the last silent under-pacing path (a forgotten stamp flies global-paced only, invisible to the boot-coverage check). A request with no RateLimit<K> extension is now rejected fail-closed; "global only" is an explicit Scope::Global, opt-out is Scope::None. Lands with the RateLimit slice (unbuilt).
2. Guarded releases the permit on a mid-stream error too — release rule becomes "the earliest of terminal frame, mid-stream error, or drop". A /history transfer that dies mid-stream must not pin one of the 5 concurrency permits while a consumer holds the errored body. Touches the Guarded body shipped in feat(net): AuthSource + Auth/SetHeaders layers + Guarded body (Slice 0 PR 3) #66 — the one concrete code change.
3. async-lock rationale re-based on the stated multi-backend goal — smol/async-std backends are a stated goal, so option R (async-lock) over P (tokio features=["sync"]) has a concrete payoff (a non-tokio stack that is genuinely tokio-free), not a "thin margin". Doc-only.
4. MockTimer lives in its own dev-only oath-adapter-net-mock crate beside the Timer contract in net-api (not net-http-mock), so the WS stack (ADR-0032/0033) shares one fake clock without an HTTP dep edge. Lands when the mock crates are built.

Scope of this PR

• ADR-0034: append-only Amendments (2026-07-04) section (docs).
• Construction-surface spec: seam refinements (docs).
• Guarded mid-stream-error release (code) — decide whether to include here or split.

Full reasoning in docs/superpowers/specs/2026-06-30-net-http-construction-surface-design.md and ADR-0034.