feat(core): support custom per-code referral splits in applyReferral and createCode

[Echolonius]

🚀 Feature: Per-Code Custom Referral Split Overrides

💡 Overview & Problem Statement

Currently, applyReferral applies the global DEFAULT_SPLIT (0.6 affiliate / 0.2 customer) or a caller-provided split option. However, top-tier affiliates or promotional campaigns often require custom commission splits stored directly on the referral code record (e.g. VIP 80% commission split vs standard 60%).

🛠️ Solution Implementation

1. Types (src/types.ts):
   • Added optional split?: ReferralSplit override field to ReferralCode.
2. Validation & Calculation (src/core.ts):
   • Updated applyReferral: uses opts.split ?? record.split ?? DEFAULT_SPLIT to evaluate commission calculations.
   • Updated createCode: accepts optional split?: ReferralSplit option.
3. Unit Tests (tests/core.test.ts):
   • Added unit test verifying applyReferral applies record.split override configured on ReferralCode.

✅ Verification

• All 20 Vitest unit tests passing cleanly.
• tsup build clean.
• Signed with GPG/SSH key.