docs: verify the downloaded Unreal archive and record the 5.2.0 pin conflict

The archive on disk is Unreal Engine 5.2.0 (CL 25360045, payload dated
2023-05-06), 19.59 GiB, ZIP central directory present and unzip -tq clean.
That contradicts the planned 5.8.x pin, so the decision, its costs, and the
extraction plan are written into Docs/setup.md rather than silently pinned.

Docs/setup.md

@@ -21,13 +21,28 @@ Anything marked **human step** needs credentials, licence acceptance, or a decis
 
 - `[x]` **Human step.** Create an Epic account, accept the Unreal license.
   Done on 2026-11-06 with the account email recorded locally; no credentials stored in the repository.
-- `[ ]` **Human step.** Acquire one pinned engine build. Two routes:
+- `[-]` **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: Linux_Unreal_Engine_5.2.0.zip, 19.59 GB, Built on 2026-11-03.
+  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.
+- `[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.
 - `[ ]` Record the exact engine version and build route in [toolchain.md](toolchain.md).
 - `[ ]` Validate that the engine editor opens and a blank project runs on Bazzite. Unreal does not support this distribution; record any workarounds.
 
+### Engine version decision
+
+The archive on disk is **Unreal Engine 5.2.0**, released May 2023, three and a half years before this project started. That is a legitimate choice — nothing in the `0.1.0` scope needs a newer feature — but it carries known costs that must be recorded before pinning it:
+
+- The bundled Linux toolchain in 5.2 targets an older glibc than Bazzite 45 ships. Project C++ compilation may need the bundled clang with older sysroot flags, or may simply fail; that has to be tested before any C++ work.
+- GPU support: 5.2 predates the driver and Vulkan paths that Blackwell cards (RTX 50 series) exercise. Vulkan RHI should still work, but shader compilation and Lumen behaviour on a 5070 Ti are unvalidated.
+- 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.
+
 ## 3. Tooling pins
 
 - `[x]` Blender 5.2 LTS (Flatpak) and MPFB 2.0.18 recorded.

Docs/toolchain.md

@@ -10,7 +10,7 @@ Audited 2026-11-06 on the development workstation.
 | --- | --- | --- | --- |
 | Bazzite | `45.20261105` (Kinoite, `bazzite-dx-nvidia`), kernel `7.2.15-ogc1.1.fc45` | `ujust update` / `rpm-ostree upgrade` | Booted and default. `44.20261026` retained as rollback. |
 | NVIDIA driver | `615.78.08`, open kernel module | Bazzite image | Verified: desktop session renders through the RTX 5070 Ti; Vulkan GPU0 is the RTX 5070 Ti on driver `615.78.08`, conformance `1.4.3.3`. |
-| Unreal Engine | `5.8.x` — pin one exact build | Epic (license required) | Not installed. |
+| Unreal Engine | `5.2.0` archive on disk (CL 25360045, `++UE5+Release-5.2`); `5.8.x` was the planned pin | Epic download, 2026-11-06 | Downloaded, not extracted, **version decision open** — see [setup.md](setup.md#engine-version-decision). |
 | Unreal Linux toolchain | engine-bundled clang | Engine source tree | Not installed. Do not substitute the system compiler. |
 | Blender | `5.2` LTS (upstream patch `5.2.3`) | Flatpak `org.blender.Blender` | Installed. |
 | MPFB | `2.0.18` (extension `mpfb`, declares Blender >= 4.2) | Blender extensions platform | Installed. Load-and-export check on 5.2 still pending. |