Draw all of a client's views in one renderer process

The adapter starts one renderer process per client, with the first engine
(again if it has exited), and attaches each engine's channel to it through a
control section (protocol version 5: Control, the -attach event). The renderer
process keeps a window, swap chain and scene per channel on one Direct3D
device, takes the views' frames in turns, and closes a view when its engine
does.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

include/t5de/protocol/channel.hpp

@@ -7,7 +7,19 @@
 
 namespace t5de::protocol {
 
-inline constexpr std::uint32_t version = 4;
+inline constexpr std::uint32_t version = 5;
+
+// One renderer process draws all of a client's 3D views. A shared memory section the adapter creates once per client
+// process, named by the renderer process's `--control <name>` argument, holds this; each engine's channel is named
+// "<name>-<n>", n counting from 0. Having created a channel, the adapter increments channels and sets the event
+// "<name>-attach" (auto-reset); the renderer process opens the channels it hasn't yet (those already gone are
+// skipped), and stops drawing a channel's view when its engine sets closing. It exits with the client.
+struct Control {
+    std::uint32_t version;
+    std::uint32_t channels; // channels created so far
+};
+
+inline constexpr wchar_t attachEventSuffix[] = L"-attach";
 
 // Which part of the client's 3D view the renderer process draws. Full is the default; the others are for comparing
 // with the client's renderer, which then draws the whole view under the renderer process's window.
@@ -18,8 +30,8 @@ enum class View : std::uint32_t {
     Compare, // over the right half, the left half's scene: both renderers side by side, showing the same
 };
 
-// A shared memory section the adapter creates for each engine, named by the renderer process's `--channel <name>`
-// argument: this header, then `capacity` bytes of data. The adapter sends the command stream (commands.hpp) through
+// A shared memory section the adapter creates for each engine (named as Control says): this header, then `capacity`
+// bytes of data. The adapter sends the command stream (commands.hpp) through
 // it in chunks: it waits for the event "<name>-consumed", writes a chunk and sets "<name>-ready"; the renderer
 // process copies the chunk out and sets "<name>-consumed" (both auto-reset; consumed starts set).
 //