fix: 修复打完boss不回血的问题 restore HP between acts using original game formula

[tianyumyum]

Problem

In headless mode, player HP is not restored when transitioning between acts. After
defeating the Act 1 Boss, the player enters Act 2 with the same HP instead of being healed.

Example from game log:

• Act 1 end: HP = 22/80
• Act 2 start: HP = 22/80 (should be 80/80 at A0, or ~69/80 at A2+)

Root Cause

Between-act healing is handled by AncientEventModel.BeforeEventStarted, which is
triggered via Godot transition callbacks during the act transition animation. In headless
mode, GodotStubs no-ops these callbacks, so the healing never fires.

Fix

Add HealBetweenActs() after both EnterNextAct() call sites, replicating the original
formula from the game engine IL:

healAmount = MaxHp - CurrentHp                                                           
if Ascension >= 2: healAmount *= 0.8                                                     
                                                                                           
- Only triggers when RoomType == Boss → next act (not between regular combats)             
- Safe against double-healing: if the engine already healed (missingHp <= 0), it no-ops