perf: eliminate per-frame heap allocs in M2 renderer; UI polish and report

M2 renderer: move 3 per-frame local containers to member variables:
- particleGroups_ (unordered_map): reuse bucket structure across frames
- ribbonDraws_ (vector): reuse draw call buffer
- shadowTexSetCache_ (unordered_map): reuse descriptor cache
Eliminates ~3 heap allocations per frame in particle/ribbon/shadow passes.

UI polish:
- Nameplate hover tooltip showing level, class (players), guild name
- Bag window titles show slot counts: "Backpack (12/16)"

Player report: CMSG_COMPLAIN packet builder and reportPlayer() method.
"Report Player" option in target frame right-click menu for other players.
Server response handler (SMSG_COMPLAIN_RESULT) was already implemented.

include/game/game_handler.hpp

@@ -718,6 +718,7 @@ public:
     // Combat and Trade
     void proposeDuel(uint64_t targetGuid);
     void initiateTrade(uint64_t targetGuid);
+    void reportPlayer(uint64_t targetGuid, const std::string& reason);
     void stopCasting();
 
     // ---- Phase 1: Name queries ----

include/game/world_packets.hpp

@@ -904,6 +904,12 @@ public:
     static network::Packet build(uint64_t ignoreGuid);
 };
 
+/** CMSG_COMPLAIN packet builder (player report) */
+class ComplainPacket {
+public:
+    static network::Packet build(uint64_t targetGuid, const std::string& reason);
+};
+
 // ============================================================
 // Logout Commands
 // ============================================================