Fix/issue referral cache invalidation

[praizeD10]

closes #162

Problem

The prediction_market contract cached HasReferrer per bettor at first bet
and never invalidated it. A user who registered with no referrer, then
later added one, would still be treated as having no referrer — referral
fees were silently returned to the bettor instead of paid to the referrer.

Solution

• Implemented the referral_registry contract (was empty) with credit() that
  checks Referrer storage directly on every call — no caching
• Removed HasReferrer cache from prediction_market entirely
• Removed DataKey::HasReferrer variant

Behavior

• User with referrer: fee goes to referrer, referrer gets bonus points
• User without referrer: fee returned to bettor (no loss)
• User who adds referrer after first bet: immediately eligible for referral fees

Files Changed

• referral_registry/src/lib.rs — full implementation (347 lines)
• prediction_market/src/lib.rs — removed cache, always invoke credit()

[Muyideen-js]

@praizeD10, the PR addresses the cache invalidation issue by removing the HasReferrer cache and checking Referrer storage directly. However, there are critical issues: 1) In prediction_market, the XLM transfer to the referral contract is missing; only invoke_contract is called, so the referral fee is never sent. 2) In referral_registry's credit(), the caller (market contract) is not authenticated via require_auth, allowing unauthorized calls. 3) No tests were added to verify the new behavior. 4) CI status is missing; please provide CI results. Please fix these issues and add tests before merging.