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/game/opcodes.hpp

@@ -243,6 +243,7 @@ enum class Opcode : uint16_t {
 
     // ---- Speed Changes ----
     SMSG_FORCE_RUN_SPEED_CHANGE      = 0x00E2,
+    CMSG_FORCE_RUN_SPEED_CHANGE_ACK  = 0x00E3,
 
     // ---- Mount ----
     CMSG_CANCEL_MOUNT_AURA           = 0x0375,