feat: add IsUsableSpell Lua API for spell usability checks

Returns (usable, noMana) tuple. Checks if the spell is known and not on
cooldown. Accepts spell ID or name. Used by action bar addons and
WeakAuras for conditional spell display (greyed out when unusable).

src/addons/lua_engine.cpp

@@ -1194,6 +1194,41 @@ static int lua_UnitCreatureType(lua_State* L) {
     return 1;
 }
 
+// IsUsableSpell(spellIdOrName) → usable, noMana
+static int lua_IsUsableSpell(lua_State* L) {
+    auto* gh = getGameHandler(L);
+    if (!gh) { lua_pushboolean(L, 0); lua_pushboolean(L, 0); return 2; }
+
+    uint32_t spellId = 0;
+    if (lua_isnumber(L, 1)) {
+        spellId = static_cast<uint32_t>(lua_tonumber(L, 1));
+    } else if (lua_isstring(L, 1)) {
+        const char* name = lua_tostring(L, 1);
+        if (!name || !*name) { lua_pushboolean(L, 0); lua_pushboolean(L, 0); return 2; }
+        std::string nameLow(name);
+        for (char& c : nameLow) c = static_cast<char>(std::tolower(static_cast<unsigned char>(c)));
+        for (uint32_t sid : gh->getKnownSpells()) {
+            std::string sn = gh->getSpellName(sid);
+            for (char& c : sn) c = static_cast<char>(std::tolower(static_cast<unsigned char>(c)));
+            if (sn == nameLow) { spellId = sid; break; }
+        }
+    }
+
+    if (spellId == 0 || !gh->getKnownSpells().count(spellId)) {
+        lua_pushboolean(L, 0);
+        lua_pushboolean(L, 0);
+        return 2;
+    }
+
+    // Check if on cooldown
+    float cd = gh->getSpellCooldown(spellId);
+    bool onCooldown = (cd > 0.1f);
+
+    lua_pushboolean(L, onCooldown ? 0 : 1);  // usable (not on cooldown)
+    lua_pushboolean(L, 0);  // noMana (can't determine without spell cost data)
+    return 2;
+}
+
 // IsInInstance() → isInstance, instanceType
 static int lua_IsInInstance(lua_State* L) {
     auto* gh = getGameHandler(L);
@@ -1622,6 +1657,7 @@ void LuaEngine::registerCoreAPI() {
         {"UnitIsEnemy",         lua_UnitIsEnemy},
         {"UnitCreatureType",    lua_UnitCreatureType},
         {"UnitClassification",  lua_UnitClassification},
+        {"IsUsableSpell",        lua_IsUsableSpell},
         {"IsInInstance",         lua_IsInInstance},
         {"GetInstanceInfo",      lua_GetInstanceInfo},
         {"GetInstanceDifficulty", lua_GetInstanceDifficulty},