Chair-Stepping Fix, Facing-Flicker Fix, Hostess/Server Standing-Side Preference

Three separate, unrelated fixes to how pawns move and stand near furniture. First: workers and guests could path directly through a chair (table seat or podium waiting chair), occupied or not — fixed by including every waiting chair (not just table seats) in the shared chair-avoidance set used everywhere a walk gets planned, including the congestion "yield" maneuver that previously ignored it. Second: every agent in the game briefly flickered to face backward for exactly one frame at every waypoint arrival, confirmed via direct simulation stepping to happen on every single arrival, not an occasional glitch — root cause was computing facing from an agent's own continuous position at the exact instant it briefly equals its just-reached waypoint; fixed by computing facing from grid-cell-to-grid-cell direction instead, which has no equivalent zero-delta instant (flicker count went from thousands per run to zero). Third: Restaurant.serverStandingPointFor now takes a side preference — the hostess prefers the side of a table closest to the podium, a server/busboy the side closest to the kitchen pass — so the two naturally spread to opposite corners of a table instead of converging on the same one.