fix: Restrict pact affinity cast speed bonus to invocation spells only

Previously (commit 62638d6), the pact affinity cast speed bonus was applied
to ALL spell casts (active, equipment, invocation). Per design intent, it
should only apply to invocation system spells.

Changes:
- invocation-system-spec.md: Update §6.2, §10.2, AC-13 to clarify bonus
  applies to invocation spells only
- gameStore.ts: Stop computing pact affinity bonus into attackSpeedMult;
  pass base 1.0 instead
- combat-actions.ts: Remove stale AC-13 comment
- combat-invocation.ts: Compute pact affinity cast speed bonus locally
  for invocation spells only, using prestige upgrade level + discipline bonus
- invocation-utils.ts: Remove computeAttackSpeedMultFromPactAffinity (no
  longer needed)

All 1235 tests pass.

docs/dependency-graph.json

@@ -1,6 +1,6 @@
 {
   "_meta": {
-    "generated": "2026-06-13T11:42:21.085Z",
+    "generated": "2026-06-13T15:26:13.488Z",
     "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."
   },
@@ -760,6 +760,7 @@
       "utils/element-cap-bonus.ts",
       "utils/element-distance.ts",
       "utils/index.ts",
+      "utils/invocation-utils.ts",
       "utils/safe-persist.ts"
     ],
     "stores/gameStore.types.ts": [],