In a notebook's room a frame goes to the relay once and the relay copies it, to everyone or
to the slots named (relay::GROUP, BROADCAST), sealed under a key of the sender's own and
numbered, with its count of broadcasts, so a repeat, a reorder, a forgery or a lost broadcast
still breaks the connection. Only a guest and its host keep a stream through the relay;
hellos, presence and Touched go to everyone, deltas to the guests holding that section.
Presence goes at most ten times a second. Touched carries each file's stamp hash, so a guest
holding that image knows the stamp without asking, and a guest that missed a change reads it
as writes from the image it holds. A room holds 64 peers and admits 120 a minute, a join told
to wait a few seconds waits, a room's byte budget counts what the relay gives out, and
/health counts bytes in and out.

examples/live_crowd.rs runs 30 guests, 15 typing, through a local relay: relay traffic falls
from about 2 MB/s each way to 0.15 in and 0.9 out, relay memory halves, and carets arrive in
a few milliseconds. Keys into one section from fifteen typists still take seconds, as each
commit waits its turn at the host.

The relay must be redeployed before clients of this version meet through it.

Assisted-by: claude-opus-5.5