feat: turn off KDE's middle-click paste

Inside a Chromium application the flag settles this on its own - with
autoscroll on, Blink stops pasting the primary selection. Everywhere else
on the desktop middle click goes on pasting, which is the thing this whole
program exists to get away from.

KWin has a switch for it, on Wayland, where the paste is a protocol the
compositor either offers or does not: EnablePrimarySelection in kwinrc.
Written with kwriteconfig, the way System Settings would, because kwinrc
has a cascade behind it. What the key said before goes in the ledger, so
disable puts it back - and a key that already said false is left alone and
not recorded, so a paste the user switched off themselves is never
switched back on.

X11 has no such switch and nothing is written there.

doc/middleclick-autoscroll.1.scd

@@ -172,6 +172,35 @@ and has a configuration file with a slot for extra arguments; that slot is
 where the flag goes. Installed as a plain package, a Flatpak or a snap, it is
 an ordinary desktop entry and needs nothing special.
 
+# MIDDLE-CLICK PASTE
+
+Inside a Chromium application the argument settles this on its own: with
+autoscroll on, Blink stops pasting the primary selection on middle click,
+because the button is doing something else now. Everywhere else on the desktop
+middle click goes on pasting, so the desktop's own switch for it is turned off
+too.
+
+KDE has such a switch, and only for a Wayland session, where the paste is a
+protocol the compositor either offers or does not:
+
+	~/.config/kwinrc, [Wayland], EnablePrimarySelection=false
+
+It is written with *kwriteconfig6*, the same tool System Settings uses, because
+kwinrc has a cascade behind it and entries a distribution can mark immutable;
+an edit that reaches past all that writes something KDE goes on to ignore. KWin
+reads the key once, while it starts, so the change is true from the next login
+on rather than straight away.
+
+What the key said before is recorded, and *disable* puts that back. A key that
+already said *false* before any of this is left alone and not recorded either,
+so a paste the user switched off themselves is never switched back on.
+
+On X11 there is nothing to switch. The primary selection is part of X itself
+and every toolkit reaches for it on its own; no one place can say no. The
+status screen leaves the row out where there is nothing to report.
+
+Turn it off under *Settings* to keep the desktop's middle-click paste.
+
 # WHAT CANNOT BE DETECTED
 
 An AppImage keeps its payload in a compressed filesystem, so there is no way to
@@ -190,6 +219,11 @@ _~/.local/state/middleclick-autoscroll/ledger_
 _~/.local/state/middleclick-autoscroll/backup/_
 	Copies of the files that are edited in place rather than shadowed.
 
+_~/.config/kwinrc_
+	KDE's own configuration. One key in it, *EnablePrimarySelection*, is
+	written when the desktop's middle-click paste is turned off, and put
+	back by *disable*.
+
 _~/.cache/middleclick-autoscroll/detect_
 	Which programs were found to be Chromium, keyed by size and modification
 	time. Safe to delete.
@@ -207,7 +241,7 @@ up at the next *apply* rather than on their own.
 
 # SEE ALSO
 
-*systemctl*(1), *flatpak*(1), *snap*(8)
+*systemctl*(1), *flatpak*(1), *snap*(8), *kwriteconfig6*(1)
 
 # AUTHORS