feat(tcp): TCPStreamer DataSources, in-process TCPStreamService, stream multiplexing and TCPStreamJoin

Add TCPStreamer/TCPStreamerClient (TCP transport behind the UDPSServerI/UDPSClientI
seam), an optional single in-process TCPStreamService serving many streams over
worker threads, multiplexed SUBSCRIBE (comma list, stream index in fragmentIdx),
and TCPStreamJoin, which merges several streams into one sample-and-hold stream
paced by a master. StreamHub takes a comma list of streams over one connection.
Includes GTests, E2E scenarios s58-s73 and docs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/superpowers/specs/2026-11-06-tcp-stream-join-design.md

@@ -0,0 +1,86 @@
+# TCPStreamService: joining streams (multiplex + merge)
+
+Date: 2026-11-06. Extends `2026-11-05-tcp-streamer-design.md` section 7.
+
+## Goal
+
+Let one consumer take several service streams through a single, cheaper path.
+Two independent mechanisms were requested ("both"):
+
+1. **Multiplexed connection**: one TCP socket carries N streams, each keeping its
+   own CONFIG, rate and timeline.
+2. **Merged stream**: the service builds one new stream whose CONFIG holds the
+   signals of several source streams, emitted at the pace of a master source
+   with sample-and-hold of the others.
+
+## 1. Multiplexed connection
+
+### Wire
+- `SUBSCRIBE` (type 5) payload becomes a comma separated list of stream names
+  (`"fast,slow"`). One name is the old behaviour. Names may not contain `,`.
+  Limits: 64 names, 4096 payload bytes, no duplicates.
+- The index of a stream is its position in the list.
+- Every server-to-client frame carries the stream index in `fragmentIdx`
+  (`totalFragments` stays 1). Single-name connections always send 0, so
+  existing clients are unchanged.
+- Any unknown name closes the whole connection (the client retries), exactly
+  like the single-name case.
+
+### Server
+- `TCPSConn` holds up to 64 stream attachments instead of one.
+- A stream's frame is built once; `Push` copies it into each subscriber's queue
+  and stamps that connection's own index into `fragmentIdx`.
+- `TCPSFrameQueue` accounts per stream: `ClientQueueDepth` DATA frames *per
+  stream*, drop-oldest of the same stream only, and a CONFIG only discards the
+  stale frames of its own stream. A fast stream can therefore never evict a
+  slow stream's frames.
+- `Flush` batches queued frames into one `writev()` (up to 16 frames), which is
+  the optimisation for the many-small-frames case; it also applies to
+  single-stream connections.
+- If a stream unregisters, every connection attached to it is closed.
+
+### Client
+- `TCPSClient` accepts `StreamName = "a,b,c"`. It demultiplexes by
+  `fragmentIdx` to one listener per stream and tracks the counter gap per
+  stream.
+- `TCPSClient::GetStream(i)` returns a `UDPSClientI` facade for stream `i`.
+  Facades reference-count `Start()`/`Stop()` of the shared connection, so
+  existing consumers (StreamHub sessions) work unchanged.
+- `TCPStreamer` / `TCPStreamerClient` DataSources are single-stream: a comma in
+  `StreamName` is rejected there.
+
+### StreamHub
+- A source with `Transport="tcp"` and `StreamName="a,b,c"` creates one session
+  per stream, all sharing one `TCPSClient`. Session labels are
+  `<Label>_<stream>` (just the stream name when no label is given).
+
+## 2. Merged stream (`TCPStreamJoin`)
+
+```
++Join = { Class = TCPStreamJoin Service = "Svc" StreamName = "all"
+          Sources = "fast,slow" Master = "fast" }
+```
+- Root-level object, like the service. It taps the source streams
+  (`TCPSStreamTapI`), and registers its own output stream with the service once
+  every source is registered. Losing a source unregisters the output.
+- CONFIG: concatenated descriptors, names `<stream>_<signal>` (<= 63 chars),
+  `timeSignalIdx` rebased, publish mode Strict, common `hrtFrequency` (sources
+  with different frequencies are refused).
+- DATA (one per master frame): `[master timestamp][source0 body][source1 body]...`
+  where a body is that source's last DATA payload without its 8-byte
+  timestamp. Nothing is emitted until every source has delivered one DATA.
+- Only Strict and Decimate sources can be merged (Accumulate batches have
+  incompatible lengths) and a source body whose size changes without a CONFIG
+  change is ignored. Held values of a slower source are stamped with the
+  master's timestamp unless that source uses a time signal.
+- Locking: join mutex, then the output stream mutex, then connection `qMutex`.
+  Notifications from the service happen outside its topology lock.
+- No extra thread: the merged frame is built on the master producer's thread.
+
+## Testing
+- GTest: queue per-stream accounting + writev partial writes; service mux
+  (index tagging, fairness, unknown name, duplicate name, comma rejection);
+  client demux/gaps/facades; join (config merge, sample-and-hold, refusal
+  cases).
+- E2E: a chain scenario with a multiplexed hub source (`tcp_service` + comma
+  list) and one with a merged stream.