Defines three client tiers (browser HTML, browser extension with
sexpr.js, Rust native client) served from the same route handlers
via content negotiation. Includes native sexp:// protocol design
over QUIC, content-addressed caching, bidirectional streaming,
self-describing schema, and implementation plan from Phase 1
(Quart content negotiation) through Phase 7 (fallback gateway).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
