fix: restore compatibility with latest Slay the Spire 2 build

[mwijffelaars]

A recent Slay the Spire 2 update changed three game APIs the mod binds to directly. Because MonoMod JIT-compiles these methods, the missing members make whole methods fail to compile at runtime — combat state reads throw MissingMethodException on CombatManager.get_IsPlayPhase(), and the shop / card-target paths break the same way.

Symptom reported in the wild (combat):

Failed to read game state: Method not found: 'Boolean MegaCrit.Sts2.Core.Combat.CombatManager.get_IsPlayPhase()'.
  at STS2_MCP.McpMod.BuildBattleState(RunState runState, CombatRoom combatRoom)

Changes

• CombatManager.IsPlayPhase was removed. Added an IsPlayPhaseCompat() helper that reconstructs "it's the player's interactive turn" from members that still exist (IsInProgress && !IsEnemyTurnStarted && !EndingPlayerTurnPhaseOne && !EndingPlayerTurnPhaseTwo) and routed all 7 references through it. It leans permissive on purpose — every action guard also checks PlayerActionsDisabled right after, which covers transient action locks.
• player.Creature.CombatState now returns ICombatState rather than the concrete CombatState. Widened ResolveTarget()'s parameter to the interface; it only uses GetCreature()/Enemies, both present on ICombatState, and CombatState implements it, so all callers still bind.
• MerchantRoom.Inventory was removed. Switched the two call sites to the new MerchantRoom.GetLocalInventory().

Verification

Built clean (0 warnings, 0 errors) against the current game assemblies, and verified in-game: combat state reads and shop both work again. The removed/renamed members were confirmed against the current sts2.dll via metadata inspection.

[mwijffelaars]

Closing as a duplicate — I hadn't seen that #117, #116, and #110 already address the same v0.107.1 API breakage (IsPlayPhase / MerchantRoom.Inventory). Thanks, and apologies for the noise!