ui: {"env": "widthClass"} is never set, so no screen can reach a compact layout #1046

Summary

{"env": "widthClass"} is documented as the window's width class (compact < 600, medium < 1200, expanded), but nothing ever sets it, so a document cannot reach a compact layout through it.

• Session::setEnvironment holds the value (include/morph/ui/interpreter.hpp:962), but nothing calls it: grep -rn setEnvironment include src finds only that definition.
• The Qt Quick frontend computes the class and hands it to FrontendConfig::onWidthClass, whose documentation says "The shell sets it as {"env": "widthClass"}" (include/morph/qt_quick/frontend.hpp:50-52, docs/spec/qt_quick/backend.md:44-45).
• morph::client::ShellOptions (include/morph/client/shell.hpp:131-157) has no way to reach the mounted session's environment, and the generic client passes .onWidthClass = {} (examples/client/main.cpp:234).
• The testkit's ScreenHarness and render smoke can't set it either.

Verification status

• Read in the code at master 842701f0d, by grep. That the value reads null at run time is inferred, not observed.
• Found by the LIMS gallery (LIMS gallery: ten LIMS screens in ui/1, and the declarative UI gaps they expose #1045). Every one of its screens has a compact layout, and none of them can reach it from the window width. The gallery works around this with a screen parameter.
• Not checked: the terminal frontend, which has no width class at all.

What would change the verdict

Close when a screen mounted by morph::client under Qt Quick reads env.widthClass as compact in a window narrower than 600 px, and follows a resize across the boundary. A test that resizes the window should show it. Reopen if the terminal frontend is expected to report a class and doesn't.