feat: implement issues #579, #580, #581, #582

[tegaReed-crypto]

#579: Add configurable min/max invoice face value bounds per risk tier

• Add AmountBounds struct to shared types
• Add set_amount_bounds/get_amount_bounds to invoice_nft
• Check bounds in mint_invoice and list_invoice

#580: Implement multi-signature repayment approval for high-value invoices

• Add RepaymentApproval struct and storage to financing_pool
• Add propose_repayment, approve_repayment, execute_repayment
• Modify repay to require approval for amounts >= threshold

#581: Add grace period logic before default marking

• Add grace_period to financing_pool initialization and storage
• Modify mark_default to check due_date + grace_period

#582: Implement invoice bundling contract (tranche)

• Create new contracts/tranche contract
• Add create_tranche, fund_tranche, get_tranche

Also fix duplicate validation functions in shared/validation.rs

closes #579
closes #580
closes #581
closes #582

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