feat: add dividend distribution and claim functionality

[syed-ghufran-hassan]

Summary

• Implement distribute_dividend to allow anyone to deposit XLM and split it proportionally among current key holders of a creator, after deducting a protocol fee.

• Implement claim_dividend for holders to withdraw their accrued dividends; resets claimable balance to zero.

• Add get_claimable_dividend view function to read unclaimed dividend balances.

• Maintain a holders list per creator, updated on key buys and sells, to efficiently iterate over current holders during distribution.

• Add new storage keys HoldersList and DividendClaimable to persist holder sets and claimable amounts.

• Extend ContractError with ZeroClaimable, NoHolders, DividendAmountZero (appended at the end).

• Add event structures and helpers (DividendDistributedEvent, DividendClaimedEvent) and publish events on distribution/claim.

• Reuse existing protocol fee configuration to deduct fees from distribution amounts.

• Update buy_key and sell_key to add/remove holders from the list.

Testing

• 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

[syed-ghufran-hassan]

Please review as incorporated comment

[syed-ghufran-hassan]

Changed incorporated