Apply active partial replacements when a meeting starts

Complex Chrome acceptance in isolated meeting 16 configured Donald Duck as partially absent from meeting start until 19:00 with Fetter Anton as the confirmed deputy. Oppmøteoversikt persisted `partially_replaced` / `partially_replacing` through refresh, but canonical run initialization made Donald the active holder and left Fetter as an idle deputy. Both subsequent poll electorates included Donald rather than Fetter. Full-day replacements in the same roster initialized correctly. Roster initialization must evaluate partial replacement intervals against the trusted meeting start/effective time and materialize the active deputy when the interval includes startup, while preserving future transitions and the five-seat/unique-occupancy invariants. Add end-to-end coverage from pre-meeting attendance state through start, participant overview, and poll eligibility.