feat(contracts): add issuance summary and claim-expiry accessor

[Nanle-code]

Closes #703

• issuance_summary(voucher_type_id): returns total_issued, max_supply, remaining (u64::MAX when uncapped), paused
• claim_expiry(voucher_id): returns expires_at_ledger, claimed, is_expired (computed live against current ledger at read time)
• Unknown ids return exists=false with zeroed fields
• 9 unit tests: success, uncapped, paused, missing, expired, claimed, supply exhaustion

Description

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Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

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Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

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📥 Commits

Reviewing files that changed from the base of the PR and between 760aa52 and 5a25475.

📒 Files selected for processing (6)

• contracts/Cargo.toml
• contracts/voucher-minter/Cargo.toml
• contracts/voucher-minter/src/lib.rs
• contracts/voucher-minter/src/storage.rs
• contracts/voucher-minter/src/test.rs
• contracts/voucher-minter/src/types.rs

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