fix(AR-004): make the channel byte counter measure the payload

`kpn::ChannelDataSize<T>` is what a channel reports as bytes pushed, and its
primary template returns `sizeof(T)`. It was never specialised in this repo —
only in a KPN example — so every message type reported its header size. Each
of them is a few vectors and a `cv::Mat` header owning megabytes on the heap,
so a message carrying a full decoded frame was reported at roughly 200 bytes
against 5.9 MB at 1080p. Four orders of magnitude.

That is not a cosmetic stat. It is the one instrument for choosing channel
capacities against a memory ceiling — the open half of AR-004 — and anyone
who read the MB/s column to size a channel was reading fiction. The gap could
not be measured with the tool that exists to measure it.

Every message embeds `Frame source`, so this is not confined to the
crop-carrying channels: the full decoded image rides the whole chain, and the
byte figure now says so.

Two things worth stating about what the number means. `cv::Mat` is
reference-counted, so one frame referenced from several messages is counted
once per reference — an upper bound on distinct bytes, and the right bound for
"what would this channel keep alive if nothing else held it", which is the
question a capacity answers. And an eof sentinel carries no image, so it costs
only its header and is not charged for one.

Declared against a forward declaration of the primary template rather than by
including <kpn/channel.hpp>, so the message definitions keep no dependency on
the framework carrying them, and any translation unit that can see these types
also sees their sizes — which is what stops one channel being instantiated
with the default while another gets the specialisation.

Verified in both directions: with the specialisations removed, three of the
five cases fail, `SceneAnnotation` reporting 40 bytes against the 37,632 its
single 112x112 crop occupies. 145/145 with them.

No behaviour change — this only corrects what is reported. Choosing capacities
against the corrected numbers is the next commit.

TRACES: AR-004 | SR-002

tests/CMakeLists.txt

@@ -28,6 +28,7 @@ add_executable(sae_tests
     test_embedding_dump.cpp
     test_audio_signature.cpp
     test_benchmark.cpp
+    test_channel_bytes.cpp
     ${CMAKE_SOURCE_DIR}/src/backends/gemm_backend.cpp
     ${CMAKE_SOURCE_DIR}/src/gallery/gallery_store.cpp
     ${CMAKE_SOURCE_DIR}/src/audio_signature.cpp