Exclude owed from the refundable balance in start_cancel

[0xdevcollins]

Without this a cancel can refund money a winner is still entitled to claim.

Done when

• Refundable balance becomes remaining_escrow - owed
• Existing behaviour preserved: partner contributions refunded in full first, then owner residual, pro-rated if short
• Test: cancel with a named unclaimed award does not refund that award

See plan 3.5.2 case 2.

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Plan: boundless-bounty-programs-implementation-plan.md

[coderabbitai]

🔗 Related PRs

#83 - feat(events): replace push-based select_winners with pull-model claim_prize for unlimited winners [open]
#90 - feat(events): pull-model prize claims for Single-release events [merged]

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