Fix 921 initialisation guard

[Peolite001]

Closes #921

Summary

• Adds an init() function to the referral contract to serve as an initialization guard.
• Implements AlreadyInitialised custom error to ensure initialization can only be called once per instance.
• Emits a ContractInitialised event upon successful initialization.
• Exposes a get_initialised() view function to allow for external verification of the contract's initialization state.
• Adds unit tests to verify proper initialization, double-initialization rejection, and validate emitted events.
• Closes Add contract initialisation guard to prevent re-initialisation attacks #921.

Testing

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
  (Note: I am leaving these unchecked for you to run locally or for the CI pipeline to verify, as my local test run was halted by a missing MSVC linker)

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

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[Chucks1093]

CI issues bro

[Peolite001]

CI issues bro

done

[Peolite001]

CI issues bro

done