GetInventoryItemsForSlot: filter by player proficiency

Mail helms were showing up for cloth classes — the function only
checked invType→slot shape, not whether the player could actually
equip the item.

Adds a proficiency gate using the engine's per-itemClass mask table
at [0x00C4D4A0], populated by the SMSG_SET_PROFICIENCY handler at
0x005E7B70. `table[itemClass] & (1 << subclass)` is the same lookup
4.3.4's Script_GetInventoryItemsForSlot does (via FUN_00553AD0 ->
DAT_00DD4D38), and the same source the engine's own tooltip red-text
coloring and cursor-drop equip check read from. Covers both armor
and weapon proficiencies in one mask lookup.

src/Offsets.h

@@ -1488,6 +1488,35 @@ enum Offsets {
     VAR_SKILL_LINE_COUNT = 0x00C0D930,
     OFF_SKILL_LINE_NAME = 0x0C,
 
+    // Player's known-skills table — separate from the DBC. Populated
+    // by the engine on login from server data; entries are pointers
+    // to per-skill records. Each record's `+0x00` is the
+    // `SkillLine.dbc` row (or `0` for header / category rows), and
+    // `+0x10` is the current skill rank (other rank/modifier fields
+    // follow). Source: `Script_GetSkillLineInfo` at `0x004D3610`,
+    // which walks this array indexed by the Lua arg.
+    VAR_PLAYER_SKILL_LIST = 0x00B7318C,
+    VAR_PLAYER_SKILL_COUNT = 0x00B731B8,
+    OFF_PLAYER_SKILL_LINE_ID = 0x00,
+    OFF_PLAYER_SKILL_RANK = 0x10,
+
+    // Per-itemClass proficiency bitmap maintained by the engine.
+    // 17 u32 entries (indexed by Item.dbc `m_class`, 0..16). Each
+    // entry is a bitmask where bit N is set if the player has
+    // proficiency in subclass N of that item class — i.e.
+    // `table[4] & (1 << 3)` = "can wear Mail" (armor=class 4,
+    // mail=subclass 3).
+    //
+    // Populated by SMSG_SET_PROFICIENCY's handler at
+    // `FUN_005E7B70`, which writes `table[itemClass] = mask`
+    // verbatim from the packet. Same source the engine itself uses
+    // for equip eligibility (the tooltip "Mail" red-text logic
+    // and the cursor-drop-on-paperdoll check both read this). Note:
+    // covers **weapons too** (class 2), making it strictly more
+    // general than walking the per-skill-line table for individual
+    // armor proficiencies.
+    VAR_PROFICIENCY_TABLE = 0x00C4D4A0,
+
     // Spell.dbc `BaseLevel` — the level a spell becomes available
     // (trainer offers it, quest reward grants it, etc.). Distinct
     // from `SpellLevel` at +0x74 which is the effective level used

src/addons/Info.cpp

@@ -115,13 +115,24 @@ int PushFieldOrNil(void *L, FieldByName_t accessor) {
 // `C_AddOns.GetAddOnName(indexOrName)` — the addon's directory name.
 // For numeric input, returns the engine's canonical casing. For
 // string input, returns the input verbatim once existence is
-// confirmed.
+// confirmed; nil for an unregistered name.
+//
+// String-input existence isn't free: `ResolveAddOnName` echoes the
+// string for the field accessors below (`GetAddOnTitle`, etc.) which
+// short-circuit cleanly on unknown names via the engine's own NULL-
+// return paths. `GetAddOnName` doesn't call any engine accessor, so
+// it'd otherwise echo any string. Probe the registry explicitly.
 int __fastcall Script_GetAddOnName(void *L) {
     const uint8_t *entry = ResolveAddOnName(L);
     if (entry == nullptr) {
         Game::Lua::PushNil(L);
         return 1;
     }
+    if (Game::Lua::Type(L, 1) == Game::Lua::TYPE_STRING &&
+        !AddOnExistsByName(reinterpret_cast<const char *>(entry))) {
+        Game::Lua::PushNil(L);
+        return 1;
+    }
     Game::Lua::PushString(L, reinterpret_cast<const char *>(entry));
     return 1;
 }

src/item/InventoryItemsForSlot.cpp

@@ -105,6 +105,47 @@ bool FitsSlot(int slot1Based, uint32_t invType) {
     return (kSlotMaskByInvType[invType] & (1u << (slot1Based - 1))) != 0;
 }
 
+// ── Proficiency gate ──────────────────────────────────────────────
+// `FitsSlot` only checks invType→slot shape compatibility; without
+// also gating on the player's proficiencies this would return (e.g.)
+// mail helms for a warlock since both are `invType=HEAD`. Modern
+// WoW's equipment-set picker filters by what the player can actually
+// equip; we mirror that.
+//
+// Source of truth: the engine's per-itemClass proficiency bitmap at
+// `[VAR_PROFICIENCY_TABLE]` (17 u32 entries indexed by `m_class`).
+// SMSG_SET_PROFICIENCY's handler at `0x005E7B70` writes the bitmask
+// for each class verbatim from the wire. Bit N of `table[class]` =
+// "player can equip subclass N of this class". Same data the engine
+// itself reads for the tooltip "Mail/Leather/Plate" red-text and the
+// cursor-drop-on-paperdoll-slot check — covers both armor and weapon
+// proficiencies in one lookup.
+//
+// 4.3.4's `Script_GetInventoryItemsForSlot` does the equivalent check
+// via `FUN_00553AD0(itemClass) & (1 << subclass)`, reading from
+// `DAT_00DD4D38` (its own copy of the same packet-fed table).
+
+bool PlayerHasProficiency(uint32_t itemClass, uint32_t subclass) {
+    if (itemClass > 16)
+        return true; // table only covers 0..16; be permissive past that
+    const uint32_t mask = *reinterpret_cast<const uint32_t *>(
+        static_cast<uintptr_t>(Offsets::VAR_PROFICIENCY_TABLE) + itemClass * 4);
+    if (mask == 0)
+        return true; // table not populated yet (pre-SMSG_SET_PROFICIENCY)
+                     // — fail open so empty-mask doesn't blank the list
+    return (mask & (1u << subclass)) != 0;
+}
+
+bool PlayerCanEquip(const uint8_t *record) {
+    if (record == nullptr)
+        return true; // missing record — be permissive, don't hide the item
+    const uint32_t classCode = *reinterpret_cast<const uint32_t *>(
+        record + Offsets::OFF_ITEMSTATS_CLASS);
+    const uint32_t subclass = *reinterpret_cast<const uint32_t *>(
+        record + Offsets::OFF_ITEMSTATS_SUBCLASS);
+    return PlayerHasProficiency(classCode, subclass);
+}
+
 const uint8_t *PeekItemRecord(uint32_t itemID) {
     auto fn = reinterpret_cast<GetItemRecord_t>(Offsets::FUN_DBCACHE_ITEMSTATS_GET_RECORD);
     auto *cache = reinterpret_cast<void *>(Offsets::VAR_ITEMDB_CACHE);
@@ -112,11 +153,14 @@ const uint8_t *PeekItemRecord(uint32_t itemID) {
     return fn(cache, itemID, &zeroGuid, nullptr, nullptr, 0);
 }
 
-uint32_t ReadInventoryType(const uint8_t *cgItem) {
+const uint8_t *ItemRecordForCGItem(const uint8_t *cgItem) {
     const int itemID = Item::ID::FromCGItem(cgItem);
     if (itemID <= 0)
-        return 0;
-    auto *record = PeekItemRecord(static_cast<uint32_t>(itemID));
+        return nullptr;
+    return PeekItemRecord(static_cast<uint32_t>(itemID));
+}
+
+uint32_t ReadInventoryType(const uint8_t *record) {
     if (record == nullptr)
         return 0;
     return *reinterpret_cast<const uint32_t *>(
@@ -132,7 +176,10 @@ void CollectIfFits(int slot, const uint8_t *cgItem, int location,
                    std::vector<Hit> &out) {
     if (cgItem == nullptr)
         return;
-    if (!FitsSlot(slot, ReadInventoryType(cgItem)))
+    const uint8_t *record = ItemRecordForCGItem(cgItem);
+    if (!FitsSlot(slot, ReadInventoryType(record)))
+        return;
+    if (!PlayerCanEquip(record))
         return;
     out.push_back({location, cgItem});
 }