Phase 8D: root README.txt + postinstall.bat cleans the C:\games root

The universal archive gains a root README.txt aimed at standalone
installers (which entry point to run, supported OSes, offline prereqs).
postinstall.bat (the TeslaConsole hook) deletes install.bat and that
README as its final step so cabinet deploys leave C:\games clean;
install.bat keeps the README as the standalone machine's docs.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/PLAN.md

@@ -355,17 +355,28 @@ XP consumes pre-rendered wallpapers.
     `RIOJoy\vendor\` (ViGEmBus installer for 10/11; RioGamepadXP.sys + INF
     for XP; **.NET 4.0 Full + KB2468871 redistributables** so an offline XP
     cabinet needs nothing else — adds ~70 MB, XP can't download anymore) +
-    `VERSION.txt` + README-DEPLOY.
+    `VERSION.txt` + README-DEPLOY (payload-internal detail doc).
+  - **`README.txt`** (zip root): written for a person installing on a
+    standalone computer — which Windows versions are supported, what the
+    two .bat entry points are and which one to run (`install.bat` for a
+    standalone machine; `postinstall.bat` is the cabinet launcher's hook),
+    that all prerequisites are bundled for offline install, and where the
+    app lands. Plain ASCII so XP-era Notepad renders it cleanly.
   - **`postinstall.bat`** (zip root, unattended): the TeslaConsole/launcher
     entry point, as today — detects the OS (`ver` → 5.1 = XP, 10.x = modern),
     installs the matching prereqs (ViGEmBus silently via the existing
     PowerShell on 10/11; .NET 4.0 + KB2468871 + driver INF on XP), and wires
-    the matching app flavor. No prompts, idempotent.
+    the matching app flavor. No prompts, idempotent. **Its final step
+    deletes `install.bat` and the root `README.txt`** — on cabinet deploys
+    the zip extracts into `C:\games`, and only launcher-managed files may
+    stay at that root (`del` tolerating already-missing files, so re-runs
+    stay idempotent).
   - **`install.bat`** (zip root, freestanding computers): same OS detection
     and prereq install, plus what a machine without the launcher needs —
     Start Menu/desktop shortcut to the right flavor's exe (still no logon
     auto-start; starting RIOJoy stays deliberate). Pure cmd.exe on the XP
-    path; may call PowerShell only on the 10/11 path.
+    path; may call PowerShell only on the 10/11 path. Leaves the README in
+    place (it's the standalone machine's documentation).
   - `pre-uninstall.bat` / uninstall mirror both scenarios. Release the one
     zip to Gitea per the established process.
 - **8E — Verification:** full suite stays net48-hosted (xUnit needs