Enforce prize floors in select_winners

[0xdevcollins]

A payout may pay more than advertised, never less. This is the guarantee that replaces the on-chain percentage commitment.

Done when

• amount >= floor[position] enforced when that position has a floor, reusing InvalidDistribution
• A position with no floor is accepted, which is how a prize is added after publish
• Test: paying below a floor is rejected; paying above is accepted

See plan 3.1 and 3.4.

---

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