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>

README.md

@@ -96,6 +96,9 @@ only produce a warning. Keep the file private (`chmod 600`) when it holds a pass
 
 ## Development
 
+- **Host build dependencies:** Qt 6 (≥ 6.5) and libmosquitto with its pkg-config file
+  (Arch: `mosquitto`, Debian: `libmosquitto-dev`); the `mosquitto` broker for the MQTT
+  integration tests. The containers (`docker/`) have everything.
 - **App identity** (app key `net.matzman.frameassistant`, tab name, executable name
   `frame-assistant`, texture size) is set in the root `CMakeLists.txt`. The executable must not
   be named like a SteamVR process such as `vrdashboard` (CMake checks this).