Replace winner_distribution with prize_floors on the event record

[0xdevcollins]

EventRecord.winner_distribution: Map<u32,u32> and the matching field on CreateEventParams become prize_floors: Map<u32,i128>: absolute advertised minimums, not percentages.

Done when

• Both types carry prize_floors
• create_event rejects when sum(floors) > total_budget (reuse DistributionMismatch)
• Over-funding is allowed: sum(floors) <= total_budget passes
• Empty map still rejects

Watch: no new error variants. contracterror caps at 50 and 48 are used.

See plan 3.4.

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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]
#96 - fix(events): namespace OpSeen by authorizing caller (follow-up to #68/#95) [merged]
#101 - fix(profile,events): replace saturating_add with checked_add + typed errors [merged]
#104 - feat(events): remove per-event participant caps, add paginated list getters [merged]

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