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/circular-deps.txt

@@ -1,5 +1,5 @@
 # Circular Dependencies
-Generated: 2026-06-13T11:42:23.525Z
+Generated: 2026-06-13T15:26:15.912Z
 Found: 4 circular chain(s) — these MUST be fixed before modifying involved files.
 
 1. 1) data/guardian-encounters.ts > data/guardian-procedural.ts