Implement `get_donor_record` view function

[ayshadogo]

User Story:
As a donor, I want to read my contribution record from the contract so that I can verify my donation on-chain.
Acceptance Criteria:

• fn get_donor_record(env: Env, donor: Address) -> Option<DonorRecord>
• Returns None if address has never donated
• Read-only, no auth required
• Returns total donated across all donations by that address

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