Read once: a link passed on is fetched from the site once, for the putter's player and the relay

Before, the member passing a page link on downloaded it twice, independently:
their mpv straight from the CDN and the relay's pulls again, with a
different format pick and without yt-dlp's cookies. The two drifted (the
putter played on while viewers stalled, or the reverse) and doubled the
connections the CDN saw. Now one SharedSource per track reads the site
once (resuming after cuts, backing off on 412/429/5xx and silence) into a
bounded buffer (64 MiB ahead, 16 behind) that serves both the putter's
mpv through a loopback endpoint and the server's pulls; the putter plays
the relay's own format pick, and the relay sends yt-dlp's cookies.
Up next's warm relay uses the same reader. Pieces tracks (HLS/DASH) keep
two fetches for now. Measured in the opt-in e2e: half the CDN traffic, no
412s under a lockout CDN, viewer seeks 0.29 s instead of 4 s there.

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

docs/DEVELOPMENT.md

@@ -94,7 +94,10 @@ Three suites, run from each package's folder:
 - `cd packages/server && dart test` (well under a minute): live servers
   on loopback: auth and version refusal, identity, takeover and grace
   (`presence_test` on a fake clock), media and relay byte serving with
-  source drops, Jellyfin relay against a fake, queue (and the shared
+  source drops, the read-once source (`shared_source_test`: its loopback
+  range answers, a cut and resume with backoff, seeks of either reader,
+  the two kept within one window, its memory bound), Jellyfin relay
+  against a fake, queue (and the shared
   countdown on a fake clock), session log,
   isolate `serve`, sharing (`share_test`: a listener attached and detached
   under the same members and Up next, the host key standing in for the
@@ -114,7 +117,10 @@ Three suites, run from each package's folder:
 
 Opt-in, skipped unless set: `JELLYFIN_TEST_*` (core `jellyfin_live` and
 server `jellyfin_relay_live` against a real Jellyfin; read their headers),
-`YTDLP_LIVE=1` (downloads yt-dlp and resolves a real tweet). Everything
+`YTDLP_LIVE=1` (downloads yt-dlp and resolves a real tweet),
+`WT_SINGLE_FETCH=1` (server `single_fetch_e2e`: a link passed on read
+once against twice, two real mpv against a fake CDN that cuts and locks
+out; minutes; `WT_SF_SCENARIOS`, `WT_SF_PLAY` narrow it). Everything
 else is offline: Jellyfin and link hosts are local fakes.
 
 Real-mpv tests: `sync_playback_e2e`, `coord_presence` (drops through a
@@ -129,7 +135,9 @@ stand-in yt-dlp and a Referer-checking CDN: the EDL, the headers and
 cookies, subtitles without danmaku, a 403, the D11 error),
 `media_switch_e2e` (commands during a switch wait and apply once, no mpv
 errors), `relay_e2e` (a dedicated server with no file of its
-own: a guest's file, links passed on or fetched by the server, a late
+own: a guest's file, links passed on (read once: the source's player on
+its loopback reader, nothing from mpv at the origin) or fetched by the
+server, a late
 source, a source that drops and resumes; a seek past what a slow source
 has sent, timed; a source that drops while a viewer seeks past
 the buffer and another joins: no seek loop, "Waiting for", all resume