feat: implement #522 #523 #524 #525 invoice enhancements

[obedebuka41-dotcom]

#522 — Cross-Invoice Split Linkage

• Add parent_invoice_id: Option to Invoice struct
• Block _release if parent is not Released/Finalised
• Detect invalid/too-deep parent chains at creation time (MAX_PARENT_DEPTH=10)
• Emit ChildInvoiceUnblocked(child_id, parent_id) on successful child release
• Add ParentInvoiceNotFinalised, CircularParentReference, ParentChainTooDeep errors

#523 — Late Payment Penalty Fee

• Add late_penalty_bps: u32 to Invoice struct
• Accept contributions within GRACE_WINDOW (86400s) after deadline
• Charge ceil(amount * bps / 10000) penalty and transfer to treasury
• Emit LatePaymentPenaltyCharged(invoice_id, payer, penalty_amount)
• Zero bps disables penalty; on-time payments unaffected

#524 — Invoice Batch Creation

• Add InvoiceParams struct mirroring create_invoice params
• Add batch_create_invoices(invoices: Vec) -> Vec
• Enforce MAX_BATCH_SIZE=50; return BatchTooLarge error if exceeded
• Emit individual InvoiceCreated per entry + one BatchInvoiceCreated
• Atomic: all succeed or transaction aborts

#525 — Recipient Share Precision Rounding

• Add calc.rs module with distribute_with_remainder (largest-remainder method)
• Guarantees sum(shares) == total for any input (no stroop lost)
• Largest fractional remainder recipients receive the extra stroop
• Fully unit-tested in calc.rs including property-based test
• _release uses distribute_with_remainder for all payouts

chore: update .gitignore with test snapshots, IDE, OS, and stellar-cli artifacts
close #522
close #523
close #524
close #525

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