feat(contract): add creator registration status, treasury address view, key balance helper, and duplicate registration rejection

[Akanimoh12]

Summary

This PR adds four contract improvements covering read-only view methods, a shared helper for safe key balance reads, and duplicate creator registration rejection with focused tests.

Closes #28, Closes #29, Closes #30, Closes #31

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Issue #28 — Read-only creator registration status method

• Added is_creator_registered(env, creator) -> bool contract method that returns whether a CreatorProfile exists for the given address.
• No state mutation; safe for indexers and read-only callers.
• Tests cover: unregistered address returns false, registered returns true, read-only idempotency, and independent creator lookups.

Issue #29 — Read-only protocol treasury address method

• Added DataKey::TreasuryAddress storage variant.
• Added set_treasury_address(env, admin, treasury) admin-only setter.
• Added get_treasury_address(env) -> Option<Address> read-only view method returning the current treasury routing target, or None if unconfigured.
• Tests cover: unset returns None, configured returns correct address, read-only idempotency, and updates after reconfiguration.

Issue #30 — Safe key balance read helper

• Added read_key_balance(env, creator) -> u32 public shared helper that reads a creator's key supply, returning 0 for unregistered creators.
• Refactored get_total_key_supply to delegate to read_key_balance, eliminating duplicated read logic.
• Missing-balance behavior is now consistent across the contract.
• All existing key supply tests continue to pass.

Issue #31 — Duplicate registration rejection tests

• Added a guard in register_creator that panics with "creator already registered" when the creator address already has a stored profile.
• Tests cover: duplicate registration with same handle panics, duplicate with different handle panics, and distinct addresses register independently.

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Verification

All three CI checks pass locally:

cargo fmt --all -- --check   ✓
cargo clippy --workspace --all-targets -- -D warnings   ✓
cargo test --workspace   ✓  (45 tests, 0 failures)

Files changed

• creator-keys/src/lib.rs — contract logic and helper additions
• creator-keys/tests/creator_registration.rs — new test file for Add read-only creator registration status method #28 and Add duplicate registration rejection tests #31
• creator-keys/tests/treasury_address.rs — new test file for Add read-only protocol treasury address method #29
• creator-keys/test_snapshots/ — auto-generated Soroban test snapshots

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