feat: add max_supply constraint to token contract [#66]

[dev-RAM11]

Closes #66

What was done

Added an optional hard cap on total token supply to the bc-forge token contract.

Files changed

• contracts/token/src/lib.rs — added MaxSupply to DataKey, MaxSupplyExceeded error, max_supply parameter to initialize, enforcement in internal_mint, and max_supply() getter
• contracts/token/src/events.rs — added emit_max_supply_set event
• contracts/token/src/test.rs — added 10 new tests covering all max supply scenarios

Changes

• initialize now accepts an optional max_supply: Option<i128> — pass None for uncapped supply
• Minting beyond the cap returns MaxSupplyExceeded error
• Cap is enforced in both mint and batch_mint
• Zero or negative cap values are rejected with InvalidAmount
• New max_supply() getter returns the cap or None if uncapped
• emit_max_supply_set event emitted when a cap is set at initialization

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