Combat: back the flavor-only monster abilities with real effects

Turn the four placeholder ability effects into working mechanics:
spell_resist halves player spell damage, reveal_action rolls the
player's next swing at disadvantage, fear_immune fizzles control
spells, and ally_buff grants an accumulating enemy attack bonus.
All four are armed by applyAbility, read by the shared resolution
primitives, and round-tripped through CombatStatuses for turn-based
suspend/resume. New branches are guarded by zero-valued state so the
auto-resolve characterization golden is untouched.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/plugin/combat_session.go

@@ -110,6 +110,13 @@ type CombatStatuses struct {
 	PlayerACDebuff     int     `json:"player_ac_debuff,omitempty"`
 	MaxHPDrain         int     `json:"max_hp_drain,omitempty"`
 
+	// Slice-4 monster-ability effects — the former flavor-only placeholders.
+	// EnemyRevealNext is a one-shot; the other three persist for the fight.
+	EnemySpellResist bool `json:"enemy_spell_resist,omitempty"`
+	EnemyRevealNext  bool `json:"enemy_reveal_next,omitempty"`
+	EnemyFearImmune  bool `json:"enemy_fear_immune,omitempty"`
+	EnemyAtkBuff     int  `json:"enemy_atk_buff,omitempty"`
+
 	// Persistent stat buffs from mid-fight !cast / !consume, accumulated as
 	// deltas against the freshly-rebuilt combatant. applySessionBuffs folds
 	// these back onto the player every round; diffTurnBuff produces them.