niri: native scratchpad via patched niri-scratchpad package

Replace the workspace-based minimize (declared 'minimized' workspace +
+1 offset + window-restore.py) with a native Sway-style scratchpad built
into niri. Ships as niri-patches/0001-scratchpad.patch + a niri-scratchpad
PKGBUILD. Mod+M stashes; Mod+Shift+M shows/toggles/cycles. Mod+1..9 no
longer offset.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pkg/niri-scratchpad/.gitignore

@@ -0,0 +1,6 @@
+src/
+pkg/
+niri/
+*.pkg.tar.*
+*.log
+0001-scratchpad.patch