Normalize signals consistently (LFO vs oscillator)

Currently we normalize the LFO to `0...1`, while the usual oscillator nodes generate a signal in `-1...1`. This was done for purely practical purposes, it makes it easy to apply amplitude modulation by just multiplying the signals, however poses some inconveniences e.g. to filter modulation which only "goes in one direction".

Ideally we should find a consistent solution here, e.g. normalizing the LFO to `-1...1` too, while rescaling the signal correspondingly in the amplitude modulation example (possibly using a hidden node? Would that be too confusing for the user?)

Start by tracing how LFO and oscillator nodes produce their signal ranges, then inspect the amplitude modulation example and the filter modulation path. The issue is complete when the signal normalization is consistent and amplitude modulation remains usable without making filter modulation one-directional.

Written by the indexing model from the issue text.