feat: implement spell queue window (400ms pre-cast)

When castSpell() is called while a timed cast is in progress and
castTimeRemaining <= 0.4s, store the spell in queuedSpellId_ instead
of silently dropping it.  handleSpellGo() fires the queued spell
immediately after clearing the cast state, matching the ~400ms spell
queue window in Blizzlike WoW clients.

Queue is cleared on all cancel/interrupt paths: cancelCast(),
handleCastFailed(), SMSG_CAST_RESULT failure, SMSG_SPELL_FAILED,
world-teardown, and worldport ACK.  Channeled casts never queue
(cancelling a channel should remain explicit).

include/game/game_handler.hpp

@@ -2725,6 +2725,9 @@ private:
     // Repeat-craft queue: re-cast the same profession spell N more times after current cast finishes
     uint32_t craftQueueSpellId_ = 0;
     int craftQueueRemaining_ = 0;
+    // Spell queue: next spell to cast within the 400ms window before current cast ends
+    uint32_t queuedSpellId_     = 0;
+    uint64_t queuedSpellTarget_ = 0;
     // Per-unit cast state (keyed by GUID, populated from SMSG_SPELL_START)
     std::unordered_map<uint64_t, UnitCastState> unitCastStates_;
     uint64_t pendingGameObjectInteractGuid_ = 0;