Fix mount stability, speed parsing, combat dismount, and self-targeting

- Fix SMSG_FORCE_RUN_SPEED_CHANGE parsing (missing uint32 field caused garbage speed)
- Always send speed ACK to prevent server stall, even on invalid values
- Defer mount model loading to next frame to avoid render-loop hang
- Compute mount height from tight vertex bounds instead of M2 header bounds
- Dismount when entering combat or casting spells while mounted
- Prevent auto-attacking yourself when self-targeted
- Leave combat when 40+ yards from target, close vendor at 15+ yards
- Pre-open X11 display for reliable mouse release in signal handlers

include/core/application.hpp

@@ -154,6 +154,8 @@ private:
     // Mount model tracking
     uint32_t mountInstanceId_ = 0;
     uint32_t mountModelId_ = 0;
+    uint32_t pendingMountDisplayId_ = 0;  // Deferred mount load (0 = none pending)
+    void processPendingMount();
     bool creatureLookupsBuilt_ = false;
 
     // Deferred creature spawn queue (throttles spawning to avoid hangs)