fix: add curse amplification to applyEnemyDefenses (spec §6.3)

The curse debuff was stored on enemies via dot-runtime.ts but the
amplification was never applied to incoming damage. Added curse
magnitude check in applyEnemyDefenses (combat-tick.ts) that multiplies
incoming damage by (1 + magnitude) for each active curse effect.

- Curse amplification applied BEFORE dodge/barrier/armse defenses
- Multiple curse effects stack multiplicatively
- Non-curse effects (burn, freeze, etc.) are ignored for amplification

Also updated spire-combat-spec.md Known Gaps table to reflect:
- Melee defense bypass fixed (issue #285)
- Curse amplification now implemented (issue #286)

Added 9 regression tests in curse-amplification.test.ts.

All 957 tests pass (50 test files).

src/lib/game/stores/pipelines/combat-tick.ts

@@ -65,6 +65,17 @@ export function applyEnemyDefenses(
 ): number {
   if (!enemy) return dmg;
 
+  // 0. Curse amplification (spec §6.3) — amplifies all incoming damage
+  let curseMult = 1;
+  for (const effect of enemy.activeEffects) {
+    if (effect.type === 'curse') {
+      curseMult *= (1 + effect.magnitude);
+    }
+  }
+  if (curseMult > 1) {
+    dmg *= curseMult;
+  }
+
   // 1. Dodge check (spec §5.2, §4.5)
   let effectiveDodge = enemy.dodgeChance;
   if (roomType === 'speed') {