Cross-Invoice Split Linkage for Coordinated Payouts

[Kingsman-99]

Description

Some business workflows require that payout from Invoice B is triggered only after Invoice A is fully funded and released. A parent-child linkage between invoices should block child invoice release until the parent is finalised.

Technical Context

Add parent_invoice_id: Option<u64> to the Invoice struct in types.rs. In release_payment inside contracts/split/src/lib.rs, if parent_invoice_id is Some, look up the parent's status and return ParentInvoiceNotFinalised if it is not Finalised. Emit ChildInvoiceUnblocked(child_id, parent_id) from events.rs when the child is released.

Acceptance Criteria

• A child invoice's release_payment is blocked until the parent is Finalised
• Circular parent references (A → B → A) are detected and rejected at creation time
• A child without a parent invoice releases normally
• ChildInvoiceUnblocked event is emitted on the first successful child release
• Max chain depth is capped to prevent deep recursion in the lookup
• All CI checks (cargo build --target wasm32-unknown-unknown, cargo test, cargo clippy) pass and the branch has no merge conflicts

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[obedebuka41-dotcom]

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@obedebuka41-dotcom's PR #567 was approved and merged by @Kingsman-99.

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