Add `.xsh` script mentioned in gitea #50

Note since it's actually `xonsh` code run with either,
- most pedantically: `xonsh ./snippets/calc_ppi.xsh`
- or relying on how shebang: `./snippets/calc_ppi.xsh`
  * an sheboom.

snippets/calc_ppi.xsh

@@ -0,0 +1,64 @@
+#!env xonsh
+'''
+Compute the pxs-per-inch (PPI) naively for the local DE.
+
+NOTE, currently this only supports the `sway`-TWM on wayland.
+
+!TODO!
+- [ ] support Xorg (and possibly other OSs as well?
+- [ ] conver this to pure py code, dropping the `.xsh` specifics
+     instead for `subprocess` API calls?
+- [ ] possibly unify all this with `./qt_screen_info.py` as part of
+     a "PPI config wizard" or something, but more then likely we'll
+     have lib-ified version inside modden/piker by then?
+
+'''
+
+import math
+import json
+
+# XXX, xonsh part using "subprocess mode"
+disp_infos: list[dict] = json.loads($(wlr-randr --json))
+lappy: dict = disp_infos[0]
+
+dims: dict[str, int] = lappy['physical_size']
+w_cm: int = dims['width']
+h_cm: int = dims['height']
+
+# cm per inch
+cpi: float = 25.4
+
+# compute "diagonal" size (aka hypot)
+diag_inches: float = math.sqrt((h_cm/cpi)**2 + (w_cm/cpi)**2)
+
+# compute reso-hypot / inches-hypot
+hi_res: dict[str, float|bool] = lappy['modes'][0]
+w_px: int = hi_res['width']
+h_px: int = hi_res['height']
+
+diag_pxs: float = math.sqrt(h_px**2 + w_px**2)
+unscaled_ppi: float = diag_pxs/diag_inches
+
+# retrieve TWM info on the display (including scaling info)
+sway_disp_info: dict = json.loads($(swaymsg -r -t get_outputs))[0]
+scale: float = sway_disp_info['scale']
+
+print(
+    f'output: {sway_disp_info["name"]!r}\n'
+    f'--- DIMENSIONS ---\n'
+    f'w_cm: {w_cm!r}\n'
+    f'h_cm: {h_cm!r}\n'
+    f'w_px: {w_px!r}\n'
+    f'h_cm: {h_px!r}\n'
+    f'\n'
+    f'--- DIAGONALS ---\n'
+    f'diag_inches: {diag_inches!r}\n'
+    f'diag_pxs: {diag_pxs!r}\n'
+    f'\n'
+    f'--- PPI-related-info ---\n'
+    f'(DE reported) scale: {scale!r}\n'
+    f'unscaled PPI: {unscaled_ppi!r}\n'
+    f'|_ =sqrt(h_px**2 + w_px**2) / sqrt(h_in**2 + w_in**2)\n'
+    f'scaled PPI: {unscaled_ppi/scale!r}\n'
+    f'|_ =unscaled_ppi/scale\n'
+)