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>

cmake/Mosquitto.cmake

@@ -0,0 +1,39 @@
+# Provides the imported target PkgConfig::MOSQUITTO (libmosquitto, the MQTT client
+# library; docs/DESIGN.md, D14/D35).
+#
+# Found through pkg-config (Debian: libmosquitto-dev, Arch: mosquitto). The Frame has no
+# libmosquitto, so package builds ship it like libopenvr_api.so: with
+# FRAMEASSISTANT_SHIP_MOSQUITTO=ON the library is installed into lib/ under its soname
+# (found through RPATH $ORIGIN/../lib), together with Debian's copyright file.
+
+option(FRAMEASSISTANT_SHIP_MOSQUITTO "Install libmosquitto into lib/ (package builds)" OFF)
+
+find_package(PkgConfig REQUIRED)
+pkg_check_modules(MOSQUITTO REQUIRED IMPORTED_TARGET libmosquitto)
+message(STATUS "libmosquitto: ${MOSQUITTO_VERSION}")
+
+if(FRAMEASSISTANT_SHIP_MOSQUITTO)
+    find_library(MOSQUITTO_LIBRARY NAMES mosquitto HINTS ${MOSQUITTO_LIBRARY_DIRS} REQUIRED)
+    get_filename_component(_mosquitto_dir ${MOSQUITTO_LIBRARY} DIRECTORY)
+    # The soname has been libmosquitto.so.1 since 1.x; the file behind it is versioned.
+    set(_mosquitto_soname libmosquitto.so.1)
+    if(NOT EXISTS ${_mosquitto_dir}/${_mosquitto_soname})
+        message(FATAL_ERROR "${_mosquitto_dir}/${_mosquitto_soname} not found")
+    endif()
+    file(REAL_PATH ${_mosquitto_dir}/${_mosquitto_soname} _mosquitto_file)
+    install(FILES ${_mosquitto_file} DESTINATION ${CMAKE_INSTALL_LIBDIR}
+            RENAME ${_mosquitto_soname})
+
+    # Debian's copyright file (same path for every architecture through multiarch).
+    set(_mosquitto_copyright /usr/share/doc/libmosquitto1/copyright)
+    if(EXISTS ${_mosquitto_copyright})
+        install(FILES ${_mosquitto_copyright}
+                DESTINATION ${CMAKE_INSTALL_DOCDIR}/third-party
+                RENAME libmosquitto1.copyright)
+    else()
+        message(WARNING "${_mosquitto_copyright} not found: package lacks libmosquitto's license")
+    endif()
+
+    # Shipped by the rule above: the Qt bundler must not copy it a second time.
+    list(APPEND VRDASH_BUNDLE_EXCLUDE "^libmosquitto")
+endif()