The serving edge: a loopback-only HTTP server over the layout [Document].
This crate is where tokio lives (design doc: engine crates stay sync + pure; the
runtime is quarantined to I/O edges). It exposes exactly ONE content endpoint (D12) and
binds 127.0.0.1 only, guarded against DNS rebinding (D6). The document is a snapshot:
serialized once at bind time, then served as immutable bytes — re-polls are answered
with a strong ETag/304 pair, which is all the UI's disconnect detection needs.