Fix buff bar: opcode merge, isBuff flag, and duration countdown

Root cause: OpcodeTable::loadFromJson() cleared all mappings before
loading the expansion JSON, so any WotLK opcode absent from Turtle WoW's
opcodes.json (including SMSG_AURA_UPDATE and SMSG_AURA_UPDATE_ALL) was
permanently lost. Changed loadFromJson to patch/merge on top of existing
defaults so only explicitly listed opcodes are overridden.

Also fix isBuff border color: was testing flag 0x02 (effect 2 active)
instead of 0x80 (negative/debuff flag).

Add client-side duration countdown: AuraSlot.receivedAtMs is stamped
when the packet arrives; getRemainingMs(nowMs) subtracts elapsed time so
buff tooltips show accurate remaining duration instead of stale snapshot.

src/game/opcode_table.cpp

@@ -698,13 +698,12 @@ bool OpcodeTable::loadFromJson(const std::string& path) {
 
     std::string json((std::istreambuf_iterator<char>(f)), std::istreambuf_iterator<char>());
 
-    // Save old tables so we can restore on failure
+    // Merge/patch on top of existing table (WotLK defaults must be loaded first).
+    // Opcodes NOT in the JSON keep their current mapping, so expansion-specific
+    // JSONs only need to list entries that differ from the WotLK baseline.
     auto savedLogicalToWire = logicalToWire_;
     auto savedWireToLogical = wireToLogical_;
 
-    logicalToWire_.clear();
-    wireToLogical_.clear();
-
     // Parse simple JSON: { "NAME": "0xHEX", ... } or { "NAME": 123, ... }
     size_t pos = 0;
     size_t loaded = 0;
@@ -746,6 +745,11 @@ bool OpcodeTable::loadFromJson(const std::string& path) {
         auto logOp = nameToLogical(key);
         if (logOp) {
             uint16_t logIdx = static_cast<uint16_t>(*logOp);
+            // Remove stale reverse-mapping for this logical opcode's old wire value
+            auto oldIt = logicalToWire_.find(logIdx);
+            if (oldIt != logicalToWire_.end()) {
+                wireToLogical_.erase(oldIt->second);
+            }
             logicalToWire_[logIdx] = wire;
             wireToLogical_[wire] = logIdx;
             ++loaded;