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).

docs/project-structure.txt

@@ -215,6 +215,7 @@ Mana-Loop/
 │   │   │   │   ├── cross-module-helpers.ts
 │   │   │   │   ├── cross-module-lifecycle-consistency.test.ts
 │   │   │   │   ├── cross-module-prestige-discipline.test.ts
+│   │   │   │   ├── curse-amplification.test.ts
 │   │   │   │   ├── discipline-math.test.ts
 │   │   │   │   ├── discipline-prerequisites.test.ts
 │   │   │   │   ├── discipline-reactivate-bug.test.ts