Relay: a member's media through the server's buffer (proto 8)

Viewers who cannot reach the media get it through the server: a file on
any member's computer, a link passed on by a member ("Through me") or
fetched by the server ("Server fetches it", password-protected servers
only), and Jellyfin items for guests without an account, sourced by
whoever put them up with their own sign-in (or a pasted link's key),
which never leaves their machine. Fixes guests on a dedicated server
without a Jellyfin login being blocked.

The source dials /r/.../source (works behind NAT) and answers pulls with
an adaptive in-flight window (2 x rate x min RTT, 256 KiB..8 MiB). The
server keeps a memory-only buffer of 1 MiB slices sized like a viewer's
mpv buffer (pinned head/tail, 64 MiB back, 256 MiB ahead), fills it with
one sequential pull at a time (blocked viewers first, newest request
only), fetches nothing while nobody watches through it, and drops it on
the next swap or stop. Measured with mpv through shaped links
(tool/relay_e2e.dart): 400 Mbit/s seeks ~0.03 s; 40 Mbit/s no stalls,
3 viewers cost the source ~1x the bytes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -169,13 +169,16 @@ Watching:
   your watched state, Continue watching and resume point follow what you
   saw (reported on start, every 10 s, on pause, resume, seek and track
   change, and when it stops).
-- Friends without an account watch through the host: the server fetches
-  the file with the host's sign-in and passes it on (only the current item,
-  only to people in the session). They show "via host" in the member list.
-  A dedicated server relays when started with `--jellyfin <file>` (see
-  [docs/ARCHITECTURE.md](docs/ARCHITECTURE.md), Jellyfin relay).
-- Without either, the app says "This needs a Jellyfin account on ..." with
-  a button to link one; it plays as soon as you are signed in.
+- Friends without an account watch through the server: whoever put the
+  item up passes it on automatically, with their own sign-in, which never
+  leaves their computer (only the current item, only to people in the
+  session). They show "via host" in the member list. When the app hosts,
+  or a dedicated server has its own login (`--jellyfin <file>`, see
+  [docs/ARCHITECTURE.md](docs/ARCHITECTURE.md), Jellyfin relay), the server
+  fetches it itself instead.
+- If nobody can pass it on (the person who put it up left), the app says
+  "No Jellyfin account here for ..." with a button to link one; it plays as
+  soon as you are signed in.
 
 Episodes: the ⏮ / ⏭ buttons next to the controls (or `<` / `>`,
 Shift+P / Shift+N) put up the previous or next episode for everyone. From
@@ -194,7 +197,24 @@ next episode. This is per person; nobody else is affected.
 
 "Change video" in the session panel (Tab) opens the same picker as the Host
 tab: the library starts on the series you are watching with the current
-season, else where you left it. File is offered to the host only.
+season, else where you left it.
+
+Anyone can put up a file from their own computer, also on a dedicated
+server: it streams from your computer through the server to everyone (the
+server keeps a few minutes in memory, so seeks and skipping the opening
+are quick). Keep the app open while it plays. For a link you choose how
+the others get it:
+- Everyone fetches it (default): each app plays the link itself.
+- Server fetches it: the server downloads it once for everyone, for links
+  that are slow or blocked for some. The server must be able to reach it,
+  and it must have a password (an open server refuses: anyone could make
+  it fetch addresses only it can reach).
+- Through me: your computer downloads it and passes it on, for links only
+  you can open.
+
+The last two need a link to a video file (not a page like YouTube) and
+never show the link to anyone. Your upload has to beat the video's
+bitrate: a 25-minute 2 GB episode is about 11 Mbit/s.
 
 ## Video quality and shaders