feat(creator-keys): resolve issues #619, #613, #583, #604

[chizzy192]

Summary

This PR addresses and resolves four related contract issues under a single consolidated change set:

1. Issue Add unit tests for holder_balance_key helper confirming output format and length #619: Added unit tests for the holder_balance_key helper verifying non-empty output, determinism, equal output length for different holders, and distinctness from other storage key types.
2. Issue Add integration test for full sell removing the holder balance entry from persistent storage #613: Added an integration test verifying that selling all held keys removes the holder_balance_key entry from persistent storage completely, decrements creator supply, returns 0 on subsequent reads, and preserves storage keys on partial sells.
3. Issue Add docs for contract storage layout and persistent key naming conventions #583: Added comprehensive documentation for contract persistent storage layout (docs/contract-storage-layout.md), key formats, data types, read/write helper functions, composite key conventions, and TTL extension thresholds.
4. Issue Add helper for writing and reading a creator's configured fee recipient address #604: Refactored creator fee recipient read and write call sites to consistently use read_creator_fee_recipient and write_creator_fee_recipient helper functions.

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Changes

• creator-keys/src/lib.rs: Updated get_creator_fee_recipient and update_creator_fee_recipient to use read_creator_fee_recipient and write_creator_fee_recipient helpers.
• creator-keys/src/test_issues.rs: Added unit tests for holder_balance_key (Add unit tests for holder_balance_key helper confirming output format and length #619).
• creator-keys/tests/full_sell_removes_holder_balance_storage.rs: Added integration tests for persistent storage entry removal after full sell (Add integration test for full sell removing the holder balance entry from persistent storage #613).
• docs/contract-storage-layout.md: Created contract storage layout reference document (Add docs for contract storage layout and persistent key naming conventions #583).

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Verification

All cargo test unit and integration tests compile cleanly and pass:

test result: ok. 14 passed (transfer_keys)
test result: ok. 13 passed (treasury_withdrawal)
test result: ok. 8 passed (batch_claim_dividend)
test result: ok. 6 passed (whitelist_window)
test result: ok. 5 passed (bonding_curve_preset)
...
Doc-tests creator_keys: ok. 0 failed

Closes #619
Closes #613
Closes #583
Closes #604