Trim the datastream crate to its dumb-pipe core: drop the frame-timing sidecar, make mux positions gap-free for accepted frames, and simplify the endpoint fanout.
- mux: defer position assignment from submit to drain so an overflowed submission no longer consumes a position (no synthetic gaps); submit now returns bool and the Mutex<Receiver> is removed since positions are assigned only to accepted frames
- endpoint/mux: switch from std::sync::mpsc to crossbeam-channel and drop per-event event_matches_request filtering — request filters now apply only to the initial catalog snapshot, and future events broadcast to all subscribers
- endpoint (DeliveryFanout): snapshot sender handles under the lock and deliver outside it via FanoutTarget/FanoutReport, so large batches or slow subscribers no longer block subscribe/snapshot control-plane ops
- emit/endpoint/producer: drop set_frame_timing_enabled/frame_timing_enabled and the Position return from submit_record/submit_text/submit_bytes, and add submit_text_owned taking owned String
- timing/lib/spec: delete the timing module and FRAME_TIME_CHANNEL/FRAME_TIME_CHANNEL_ID/FrameTimeSample re-exports (including the auto-registered timing channel in ChannelCatalogState) and renumber the DATASTREAM_SPEC.md section references across frame/ingest/store/mux
- tests: remove the 567-line shared datastream_support/mod.rs harness
Signed-off-by: Zachery Aaron Shores-Chmielewski <zacheryasc@gmail.com>
