Fix the nested guard: it answered wrong for a real session, and missed a case

The guard added earlier tonight asked "is WAYLAND_DISPLAY or DISPLAY set".
Both Wayland backends call set_var("WAYLAND_DISPLAY", ...) on themselves the
moment they bind their own socket, so after startup that question answers
"yes" for a real udev session too. Every publish on the config-reload path
runs after that point, which means a live session that reloaded its config
would have stopped publishing its own GTK settings - the exact opposite of
what the guard is for. It also treated srdwm-x as nested, because an X11
session naturally has DISPLAY set, even though srdwm is that display's
window manager and not a client of anything.

Nestedness is now read once, at startup, before any backend is up, and
only the Wayland backend can be nested. A test pins the second half.

The same reasoning applies to window_memory::save_all, which had no guard
at all: a nested instance shares HOME with the session it runs inside, so
dragging a test window would overwrite where that application opens in the
real session - a 1280x800 test window's position applied to a 3840x1080
desktop. Loading stays unconditional and deliberate: honouring what a real
session remembered is right, writing back over it is not. That side reads
srdwm_wayland::running_nested, recorded by connect at the moment it picks
the winit backend, since the environment can no longer be asked afterward.

Verified: a nested run with a scratch config left both the stylesheet and
window-memory.json untouched (md5 before and after, and no test window's
app-id in the store).

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