Find libmosquitto and ship it in release packages (iteration 2, step 1)

cmake/Mosquitto.cmake finds it through pkg-config; with
FRAMEASSISTANT_SHIP_MOSQUITTO=ON (all package presets) it installs
lib/libmosquitto.so.1 and Debian's copyright file and excludes the
library from the Qt bundler (D35). The build containers get
libmosquitto-dev, the test container the mosquitto broker.

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

docs/ARCHITECTURE.md

@@ -153,6 +153,9 @@ sequenceDiagram
   - `libopenvr_api.so` next to the executable
   - install rules and registration for Qt bundling
   - the check against SteamVR process names
+- **`cmake/Mosquitto.cmake`** (app, not platform) finds libmosquitto through pkg-config. With
+  `FRAMEASSISTANT_SHIP_MOSQUITTO=ON` (all package presets) it installs the library into `lib/`
+  under its soname plus Debian's copyright file, and excludes it from the Qt bundler.
 - **App identity** (root `CMakeLists.txt`): `project()` name and version, `VRDASH_APP_KEY`,
   `VRDASH_DISPLAY_NAME`, `VRDASH_EXECUTABLE_NAME`, texture size. CMake generates from them
   `AppConfig.h` and the SteamVR manifest, with `binary_path_linux_arm` for ARM builds and