feat(contract): [SC-009] Prevent Double Minting Per Slot

[KayProject]

Summary

Implements SC-009 — prevents any slot from being minted more than once in the ChronoPay contract.

Changes

• Added Minted(u32) variant to DataKey enum to track minted slots

• Updated mint_time_token to check storage before minting and panic with "slot already minted" if the slot has already been minted

• Added test_prevent_double_mint_same_slot test using #[should_panic] to verify the guard works

Security Notes

• Guard uses instance storage — same ledger scope as all other contract state

• Panic is deterministic and immediate — no partial state is written on double-mint attempt

• Slot ID is scoped per u32 key — no cross-slot interference

Test Coverage

• Existing tests (test_hello, test_create_time_slot_auto_increments, test_mint_and_redeem) unchanged and unaffected

• New test confirms second mint on same slot reverts as expected

Closes #9

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