docs: record the verified 5.8.4 archive and the extraction target

Docs/setup.md

@@ -24,8 +24,8 @@ Anything marked **human step** needs credentials, licence acceptance, or a decis
 - `[-]` **Human step.** Acquire one pinned engine build. Two routes:
   - **Launcher build**: install the Linux Epic Games Launcher, download the pinned engine version, install to the DATA volume, and disable auto-update so the pin holds.
   - **Source build**: clone the pinned `release` branch from the Epic Games GitHub organisation, then `./Setup.sh`, `./GenerateProjectFiles.sh`, `make -j16`. Budget 120–200 GB and a multi-hour first build on 16 threads.
-  Downloaded 2026-11-06 to `/run/media/system/DATA/Downloads/Linux_Unreal_Engine_5.2.0.zip`: 21 036 632 443 bytes (19.59 GiB), ZIP central directory present, `Engine/Build/Build.version` reports **5.2.0**, changelist 25360045, `++UE5+Release-5.2`, payload dated 2023-05-06.
-  Not yet extracted, and the version conflicts with the 5.8.x target in [toolchain.md](toolchain.md) — see "Engine version decision" below.
+  **5.8.4** downloaded to `/run/media/system/DATA/Downloads/Linux_Unreal_Engine_5.8.4.zip`: 39 819 212 076 bytes (37.08 GiB), ZIP central directory present with 299 959 entries, `Engine/Build/Build.version` reports **5.8.4**, changelist 58902052, `++UE5+Release-5.8`. Extracting to `/run/media/system/DATA/Engines/UE_5.8.4`.
+  The earlier 5.2.0 archive (19.59 GiB, CL 25360045) remains on disk as a fallback only.
 - `[x]` Verify the archive end to end before extracting. `unzip -tq` on 2026-11-06: "No errors detected", exit 0.
 - `[ ]` Extract to the DATA volume and record the install path. Plan: `mkdir -p /run/media/system/DATA/Engines/UE_5.2 && cd $_ && unzip -q <zip>`; engine root becomes `/run/media/system/DATA/Engines/UE_5.2/Engine`. Roughly 55–60 GB extracted, so keep it off the repository volume tree.
 - `[ ]` Resolve the engine version: 5.2.0 is what is on disk, 5.8.x was the planned pin. Record the decision and its reasons.
@@ -41,7 +41,7 @@ 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.
 
-**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.
+**Decision taken 2026-11-06, archive delivered 2026-11-07:** the current stable release is **5.8.4**, and that is what will be pinned. The 5.2.0 archive stays only until 5.8.4 launches on Bazzite, then gets deleted; two engines on disk invite accidental mixing. The 5.2.0 costs listed above are why the newer build was preferred.
 
 ## 3. Tooling pins