Client effect interpolation

LivingRef::effect_blend_factor(host, effect, partial) reads the local player's client effect blend. A newly blended ordinary effect reads 0 before its first effect tick, then interpolates to its post-countdown target each client tick: 1 while active, 0 once expired. A finite effect that expires on its first tick never rises; an infinite effect still has the initial interpolation. An unblended addition starts at its target immediately; replacing an existing effect keeps its blend. Darkness and nausea use their existing longer fades. Pass the render partial tick unchanged: the read does not clamp it. A missing effect reads Some(0.0); an unknown effect, stale handle or entity other than the local player reads None. Other visible entities' effect blends are not yet supported.