Add get_mint_timestamp read method

[Daverisky-max]

Fixes #281 - Add get_mint_timestamp read method

Summary

This PR adds a read method to return the mint timestamp for a given user-period as an Option<u64>, resolving issue #281.

Acceptance Criteria

• Read Method: Implemented get_mint_timestamp(e: Env, user: Address, period: u64) -> Option<u64>.
• Tests: Added test_get_mint_timestamp_exists and test_get_mint_timestamp_missing testing both scenarios in src/test.rs.
• Documentation: Explicitly documented that the returned timestamp represents ledger time, not wall-clock time, and returns None if no mint has occurred.

Notes for Reviewers

• The baseline test suite (cargo test) failed to compile prior to these changes due to a dependency resolution error in soroban-env-host v21.2.1 (ChaCha20Rng: ed25519_dalek::rand_core::CryptoRng trait bound not satisfied). The changes introduced here follow existing storage patterns and should be structurally sound once the dependency tree compiles.
• Extracted the timestamp as a u64 based on the existing WrapRecord { timestamp: u64 } struct implementation.
• Pre-existing clippy warnings in src/mint.rs regarding manual range bounds were left untouched as they were out of scope.

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