feat: milestone-linked partial collateral release as milestones complete

[shakurJJ]

Summary

Supplier collateral is locked in full at shipment creation and only ever released at cancellation/completion. This proposes releasing collateral incrementally as milestones confirm, proportional to shipment completion.

Proposed Behaviour

• When a milestone confirms, release a proportional share of the locked collateral back to the supplier based on that milestone's percentage of the shipment.
• Any remaining collateral is released (or forfeited per existing dispute-resolution rules) at final completion/cancellation, unchanged from today.
• Opt-in via a shipment-level flag so existing shipments and integrations are unaffected by default.

Acceptance Criteria

• Collateral release is proportional to confirmed milestone percentage when the flag is enabled
• Default (flag disabled) behaviour is unchanged from today
• Disputed milestones do not trigger early collateral release
• Tests cover partial release across multiple milestones and the disabled-flag path

[Mubking-dev]

@Mubking-dev has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello maintainer, I can take this on.
The collateral is locked until final completion right now. I’ll add support for incremental release so a percentage is released when a milestone is confirmed, based on milestone weight. The rest will still release on final completion, cancellation, or dispute. I’ll make it opt-in per shipment so existing behavior isn’t broken, and add tests for partial release, final release, and forfeiture.

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