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/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. |