PitchWalker was classified as a generator but always consumed the
incoming pattern's triggers. Reclassify as a processor so it modifies
the pattern produced by an upstream generator (clonePattern instead of
createPattern). Add reEvalOnLoop flag on the Primitive base class; when
set, the sequencer re-evaluates the chain at each loop start so
stateful primitives produce evolving patterns across loops.
Generators can now have independent step counts. When merging patterns
of different lengths, both are tiled to their LCM. Chain exposes
setGeneratorStepCount/getGeneratorStepCount for polyrhythmic control.
