docs: pin QGIS 4.2.3 and compute the projected origin with PROJ

QGIS stable is 4.2.3 on the current KDE runtime; the stale 3.28.9 lts install
stays flagged for removal. PROJ 9.9.0 from that flatpak turns the recorded
origin into E 376515.389 / N 3774224.083 (NAD83 UTM 11N), with six control
points whose projected spans reproduce the OSM footprint measurements. Engine
decision recorded: 5.8.4 is downloading, 5.2.0 stays only as a fallback.

Docs/setup.md

@@ -41,13 +41,12 @@ The archive on disk is **Unreal Engine 5.2.0**, released May 2023, three and a h
 - No engine fixes from 5.3 through 5.8, and no compatibility with content or plugins authored for newer versions.
 - Bazzite is not a supported Unreal distribution in any version, so this risk compounds rather than introduces.
 
-Alternative: fetch the current stable release instead, and keep the 5.2.0 archive as a fallback until the newer one is validated. Either way, one version gets pinned in [toolchain.md](toolchain.md) and the other is deleted, because a second engine on disk invites accidental mixing.
+**Decision taken 2026-11-06:** fetch the current stable release — `Linux_Unreal_Engine_5.8.4.zip` is downloading — and keep the verified 5.2.0 archive only as a fallback until 5.8.4 launches. Once 5.8.4 validates, delete the 5.2.0 archive and pin 5.8.4 in [toolchain.md](toolchain.md); two engines on disk invite accidental mixing.
 
 ## 3. Tooling pins
 
 - `[x]` Blender 5.2 LTS (Flatpak) and MPFB 2.0.18 recorded.
-- `[-]` Install QGIS and record its version.
-  The flathub `lts` branch installed **3.28.9** on an end-of-life Qt5 runtime — not an acceptable pin. Mirror the upstream LTR instead: `flatpak install flathub --user org.qgis.qgis//stable`, and if that is also stale, move QGIS into a distrobox container from the Fedora package. Details in [environment.md](environment.md).
+- `[x]` Install QGIS and record its version. QGIS **4.2.3**, Flatpak `stable`, runtime `org.kde.Platform 6.11`, bundled PROJ `9.9.0`. The stale `lts` 3.28.9 install is still beside it and should be uninstalled.
 - `[ ]` Install Blosm in Blender, record its version, and confirm it loads on Blender 5.2. Fall back to a side-by-side Blender 4.5 LTS only for the blockout step if it does not.
 - `[ ]` Confirm the geo tool Python environment (venv with `pyproj`, `GDAL`, `shapely` from the QGIS or distrobox environment) and record it.