Extend TTL for ReferralCount/ReferralEarnings (read + write bump)

[Phantomcall]

Extend TTL for ReferralCount/ReferralEarnings (read + write bump)

Issue

Closes #73

Summary

register_referral writes ReferralCount and credit writes ReferralEarnings
to persistent storage, but neither entry ever had its TTL extended. If a
referrer was inactive for the TTL window, both entries expired and were deleted,
silently losing the referrer's historical referral record (get_referral_count
/ get_earnings would return 0).

This PR applies the same storage-lifecycle strategy already used by the
leaderboard contract: a write-bump on every set and a read-bump on
every get, keeping both counters alive for at least TTL_BUMP ledgers
(~6 months) while they are still in use.

Changes

referral_registry/src/lib.rs

• Added the TTL constants already used by leaderboard:

  const TTL_BUMP: u32 = 3_153_600;   // ~6 months of ledgers
  const TTL_HIGH: u32 = 6_307_200;   // ~1 year of ledgers

• register_referral: after incrementing ReferralCount(referrer), call
  env.storage().persistent().extend_ttl(&DataKey::ReferralCount(referrer), TTL_BUMP, TTL_HIGH).
• credit: after updating ReferralEarnings(referrer), call
  extend_ttl(&DataKey::ReferralEarnings(referrer), TTL_BUMP, TTL_HIGH).
• get_referral_count / get_earnings: after reading, call extend_ttl so
  merely querying a referrer's stats keeps their entry alive (read-bump).

referral_registry/src/tests.rs

• Added test_referral_ttls_are_extended which asserts that ReferralCount
  and ReferralEarnings have a TTL >= TTL_BUMP after the write path and after
  a read, proving the bump happens.
• Regenerated test_snapshots/... fixtures.

Pre-existing breakages fixed to make the suite runnable

While implementing this fix I found that main did not compile or run its own
referral_registry tests. These were unrelated to #73 but had to be resolved to
verify the change; they are called out here for transparency:

1. setup() called leaderboard_client.set_token(...) — the leaderboard
   method was renamed to set_token_contract(...). Fixed the test helper call.
2. register_referral invoked reward_bonus(...) on the leaderboard — that
   entry point no longer exists; the leaderboard now exposes add_bonus_pts( caller, user, pts) (points only; the welcome-bonus PULSE mint is handled by
   the leaderboard's own reward path per "Lever G"). Updated the cross-contract
   call accordingly and removed the now-unused WELCOME_BONUS_TOKENS constant.
3. test_welcome_bonus asserted the referral contract minted 1 PULSE to the
   user — under the current leaderboard design the referral contract no longer
   transfers tokens on registration, so the token assertion was removed (points
   assertion retained).

Scope

In scope: ReferralCount / ReferralEarnings TTL lifecycle in
referral_registry/src/lib.rs and the accompanying test.
Out of scope: a full storage-rental/keeper redesign (tracked separately as
issue #9) — the read/write bump keeps active referrers' history alive, which is
the pragmatic fix requested here.

Acceptance Criteria

• ReferralCount and ReferralEarnings no longer silently expire while in
  use (TTL extended on every read and write).
• New TTL test added and passing.
• cargo test -p referral_registry passes (16/16).

Verification

• cargo test -p referral_registry — 16 passed; 0 failed.