Fix issue 274

[razeprasine]

Summary

• Removed duplicate transfer_keys entrypoint — the old version with non-standard parameter order (env, from, to, creator, amount) was replaced by the canonical (env, creator, from, to, amount) version that follows the contract's API convention. The duplicate caused 9 compilation errors from the #[contractimpl] macro.
• Added sequential creator-supply test — test_creator_supply_accumulates_across_sequential_buys in creator_supply.rs performs 3 sequential buys, reads get_creator_supply after each, and asserts cumulative totals match.
• Fixed peer_to_peer_transfer_supply_regression.rs — updated transfer_keys call site to use the new parameter order.

Testing

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• All transfer_keys tests pass: 6 unit + 12 integration + 1 regression
• All creator_supply tests pass: 4/4 (3 existing + 1 new)
• cargo check compiles cleanly

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

Closes #274