feat(creator-keys): add transfer entrypoint and enforce unique holder…

[N-thnI]

… count state invariants

Summary

Description

This PR implements a direct key transfer entrypoint (transfer_key) to the contract surface and ensures that unique holder_count metrics adjust accurately based on pre- and post-transfer balances.

Scope of Changes

• Contract Surface Modification: Introduced a lightweight transfer_key function utilizing existing balance and dividend-settlement checkpoints.
• Holder Count Tracking: - Decrements holder_count when a sender transfers their entire remaining balance.
  • Increments holder_count when a recipient acquires keys for the first time.
  • Keeps holder_count unchanged on partial transfers where the sender retains assets.
• Targeted Unit Tests: Added explicit state assertions verifying all three core edge cases.

Verification Checklist

• Full transfer out decrements unique holder count.
• First-time recipient increments unique holder count.
• Partial transfer leaves unique holder count unaffected.
• Changes isolated exclusively to the transfer pipeline and associated tests.

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

closes #422

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