Add whitelist early access window so creators can reward their existing community before public launch

[Johnpii1]

Summary

Add WhitelistConfig and WhitelistStatus contract types, new DataKey::Whitelist storage key, and MAX_WHITELIST_SIZE = 500 constant for registration-time validation.

Append WhitelistOnly and WhitelistTooLarge ContractError variants and extend the register_creator ABI with an optional whitelist_window: Option parameter; store the whitelist immutably at registration and validate size ≤ 500.

Enforce whitelist-only buys during the configured window by adding assert_whitelist_buy_allowed into buy_key, plus helpers read_whitelist_config, whitelist_expires_at, and is_whitelist_window_active to compute active/expiry state.

Add get_whitelist_status(creator) view returning { active: bool, expires_at_ledger: u32, remaining_ledgers: u32 } and extend TTL handling to keep whitelist metadata alive alongside other creator state.

Update all test call-sites to the new register_creator ABI and add a new integration test file creator-keys/tests/whitelist_window.rs covering: non-whitelisted rejection during window, whitelisted buy success, public buy after expiry, status active/expired checks, over-500 rejection, and no-whitelist immediate public buy.

Testing

Ran formatting check with cargo fmt --all -- --check and linting with cargo clippy --workspace --all-targets -- -D warnings, both succeeded.

Ran the new suite cargo test --test whitelist_window and all whitelist tests passed (6 passed; 0 failed).

Ran the workspace test suite with the long-running regression test skipped via cargo test --workspace -- --skip test_flat_buy_price_lower_than_linear_at_supply_10000, and the run completed successfully (all executed tests passed).
Verified unit/integration test updates compile and the contract behaves as expected for the new whitelist flows (no regressions observed in the executed test subsets).

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace

Checklist

• Linked issue or backlog item
• Added or updated creator-keys unit/integration tests for every changed contract behavior, including failure paths for new or reachable ContractError variants
• Ran cargo fmt --all -- --check, cargo clippy --workspace --all-targets -- -D warnings, and cargo test --workspace, or explained exactly why a command was not run
• Reviewed persistent storage changes against docs/storage-key-invariants.md; any storage layout change includes a migration/backward-compatibility note
• Confirmed event names, topic order, payload field order, and field meanings remain compatible with docs/contract-event-conventions.md, or documented the breaking change and versioning plan
• Updated docs for any changed public contract interface, read-only method, event schema, storage behavior, fee logic, or deployment workflow
• Scope stays limited to one contract concern and does not include unrelated formatting, lockfile, generated artifact, or dependency changes

close #518