Add runtime MainMenu injection and docs

Introduce a C++ runtime injection that adds a Multiplayer row to the Fallout 4 main menu instead of patching MainMenu.swf. Adds F4TMainMenuInject.h/cpp implementing injection, ITEM_PRESS handling, accept/button hooks, resync tasks and logging; integrates installation by calling MainMenuInject::Install() from PrismaUI init. Update PrismaUI to use the new helper and remove a duplicate IsMainMenuOnMainPanel implementation. Documentation updated to deprecate FFDec SWF patching (new docs/MainMenu_Injection.md, note in FFDEC guide, and setup.md) and restore-vanilla-mainmenu.ps1 messaging updated to instruct using the runtime injection.

docs/FFDEC_MainMenu_Multiplayer_Guide.md

@@ -1,7 +1,11 @@
 # FFDec guide — add Multiplayer button to MainMenu.swf
 
+> **Deprecated:** Use C++ runtime injection instead. See [`MainMenu_Injection.md`](MainMenu_Injection.md). Do not deploy a patched `MainMenu.swf` for Commonwealth Online.
+
 Step-by-step instructions for adding the **Multiplayer** main-menu item using [JPEXS FFDec](https://github.com/jindrapetrik/jpexs-decompiler/releases), without breaking **Settings → Gameplay / Display / Audio**.
 
+This guide is kept for historical reference only.
+
 ---
 
 ## Before you start

docs/MainMenu_Injection.md

@@ -0,0 +1,61 @@
+# MainMenu Multiplayer — C++ runtime injection
+
+Commonwealth Online adds a **Multiplayer** row to the Fallout 4 title-screen main menu at runtime. No SWF patching is required.
+
+## Requirements
+
+- Fallout 4 launched via **F4SE**
+- `CommonwealthOnline.dll` in `Data/F4SE/Plugins/`
+- **PrismaUI_F4** and the server browser HTML view installed (for the overlay)
+- **Vanilla** `MainMenu.swf` — or no loose override at all (game loads from `Fallout4 - Interface.ba2`)
+
+## Do not ship a patched SWF
+
+| Do | Don't |
+|----|--------|
+| Use vanilla `MainMenu.swf` (61,959 bytes) or no loose file | Deploy an FFDec-patched `Interface/MainMenu.swf` |
+| Let the plugin inject the row and handle clicks | Re-import the full `MainMenu.as` script in FFDec |
+
+A patched SWF can break **Settings → Gameplay / Display / Audio**. The C++ injection path keeps vanilla bytecode intact.
+
+To restore vanilla from the game archive:
+
+```powershell
+.\tools\restore-vanilla-mainmenu.ps1
+```
+
+## How it works
+
+1. **List injection** — After `StartMenuBase::InitMainList` builds the menu, the plugin inserts `{ text: "Multiplayer", index: 15 }` after the **Load** row (index 4). A fallback task re-injects if ActionScript rebuilds the list without calling the native hook (for example after deleting the last save).
+2. **Selection handling** — A Scaleform `ITEM_PRESS` listener intercepts index `15` on the title screen (`MAIN_STATE`, not pause menu) and opens the PrismaUI server browser. Vanilla `onMainListItemPress` is not run for that row.
+3. **Browser lifecycle** — Existing `WatchMainMenuState` hides the browser when leaving `MAIN_STATE`. **F9** still toggles the browser.
+
+## Test checklist
+
+| Case | Expected |
+|------|----------|
+| Title screen main list | **Multiplayer** appears after **Load** |
+| Pause menu | No Multiplayer row |
+| Click Multiplayer | Server browser opens; no SaveLoad panel flicker |
+| Accept / gamepad A on Multiplayer | Same |
+| Settings → Gameplay / Display / Audio | No crash |
+| Settings → Controls | Works |
+| Open Multiplayer → Settings | Browser closes immediately |
+| Delete last save / list rebuild | Multiplayer row reappears |
+| F9 | Still toggles browser |
+| No loose `MainMenu.swf` | Still works (BA2 vanilla) |
+
+## Log checks
+
+In `CommonwealthOnline.log`, look for `[MainMenuInject]` lines:
+
+- `InitMainList hook and MainMenu injection tasks installed.`
+- `Injected Multiplayer row after Load.`
+- `Registered ITEM_PRESS listener ('…').`
+
+## Deprecated approach
+
+The FFDec SWF patch guide is superseded by this document:
+
+- [`FFDEC_MainMenu_Multiplayer_Guide.md`](FFDEC_MainMenu_Multiplayer_Guide.md) — historical reference only
+- [`PATCH_MainMenu_Multiplayer.md`](../Interface/exported/PATCH_MainMenu_Multiplayer.md) — historical reference only

docs/setup.md

@@ -26,6 +26,10 @@ Dedicated test save
 
 Avoid testing with a full modlist while developing the core systems.
 
+## Main menu
+
+Do **not** deploy a patched `Interface/MainMenu.swf`. Commonwealth Online injects the **Multiplayer** row at runtime via the F4SE plugin. Use vanilla `MainMenu.swf` or no loose override. See [`MainMenu_Injection.md`](MainMenu_Injection.md).
+
 ## Version Tracking
 
 Record the exact versions used: