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/dependency-graph.json

@@ -1,6 +1,6 @@
 {
   "_meta": {
-    "generated": "2026-06-06T14:50:38.455Z",
+    "generated": "2026-06-06T15:33:44.741Z",
     "description": "Import dependency graph for src/lib/game. Keys are files, values are arrays of files they import.",
     "usage": "To find what a file affects, search for its path in the VALUES. To find what a file depends on, look at its KEY entry."
   },