Derive grant milestone payments from the awarded amount

[0xdevcollins]

grant::claim_milestone currently computes winner_distribution.get(position) * total_budget / 100. With floors replacing the distribution, the payment derives from the amount carried on the award instead.

Done when

• Milestone payment is the awarded amount divided by milestone count
• The final milestone still absorbs the remainder exactly as today
• Dedicated tests on the Multi release path, including the final-milestone remainder case

See plan 3.5.2 case 3. This is a real change to a pillar outside bounties.

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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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