Function evaluate_floor 

pub fn evaluate_floor(candidates: &[FloorCandidate]) -> FloorDecision

Read the floor from the current set of contending candidates (ADR-062 D4).

• No candidates → FloorState::Open.
• A hard interrupt present → FloorState::Locked, the interrupting candidate wins with f32::MAX (preempt + cancel generation).
• One candidate → FloorState::Held.
• Several candidates → FloorState::Contended (ordered by urgency), the top scorer is the winner.

Ties are broken by input order (earliest-listed wins), so callers should supply candidates oldest-wait-first for deterministic fairness.