docs(spec): sync Client + httpx2-aligned Async* rename

Adds a sync Client to httpware with full parity to AsyncClient (typed
decoding, middleware, Retry/Bulkhead, streaming). Aligns naming with
httpx2: sync becomes the unprefixed default, async carries the Async*
prefix. Drops Retry(attempt_timeout=...) from both worlds. Two PRs
(rename then sync), one release.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: lesnik512