SetCVar and RegisterCVar read their arguments as 1.12.1 does: a number as %.14g text, "0" for a value with no string form, the reference usage text, realmList/realmName/scriptMemory read-only in the world, and CVAR_UPDATE fired inside the call for every token (#557)

`SetCVar` and `RegisterCVar` now read their arguments as the reference's bindings do (`0x488c10`, `0x488b00`):

- a number is stored as Lua's `%.14g` text (`luaV_tostring` `0x6f7c80`), so the stock Sound slider's 0.4 stores `"0.40000000596046"`;
- a value with no string form (nil, absent, boolean, table) stores `"0"` (`0x488c98`, `0x488b6f`), so unticking a stock Sound options box no longer raises and music can be switched off there;
- the reference's usage texts (`0x84235c`, `0x8422e8`);
- `realmList`, `realmName` and `scriptMemory` are read-only to Lua `SetCVar` in the world (`0x48f566`-`0x48f584`), and host writes still land;
- `CVAR_UPDATE` fires inside the call for every string token, changed or not (`0x488cad`-`0x488cd5`, `SignalEvent2` `0x703f50`).

Verified: 7 new VM tests, a stock Sound window test off the chain and a world-entry read-only test, each failing with the change undone; `scripts/gates.sh` green. Built on macOS; no platform seam touched.

Fixes #167
Fixes #139
Fixes #547

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/benilla-app/src/ui_script/lifecycle.rs

@@ -306,6 +306,13 @@ pub(crate) fn load_ingame_ui_on_world_entry(world: &mut World) {
         .and_then(|r| r.realm.as_ref().map(|r| r.name.clone()))
         .unwrap_or_default();
     script.set_realm_name(&realm);
+    // `CGGameUI::InitializeGame` flags these read-only before `UI_Init` loads anything
+    // (`0x48f566`-`0x48f584`), so an in-game `SetCVar` of one raises, logout handlers included;
+    // `ShutdownGame` clears them (`0x491240`), and here the flags die with this VM. The host's
+    // own realm writes are engine writes, which the flag does not stop.
+    for name in benilla_ui::script::IN_WORLD_READ_ONLY_CVARS {
+        script.set_cvar_read_only(name, true);
+    }
     // The player too, before the addon walk. The record first: the reference's
     // `UnitName`/`UnitRace`/`UnitClass`/`UnitSex` read only a copy of the char-enum row made at the
     // Enter World commit (`0x5abd9e`) and never cleared, and each rebuilt VM is told once. The