Per-generator tempo ratio for Link following

[benmca]

Problem

When NoteGeneratorThread follows an Ableton Link session, _set_generator_tempos() walks the full generator tree and sets every rhythm-notetype Itemstream to the same BPM. Any per-generator tempo differences set at construction time are overwritten on the first Link tempo change.

This means you can't have a child generator running at half tempo (or any other ratio) relative to the Link session — all voices are locked to the same absolute BPM.

Proposed solution

Add a tempo_ratio attribute to NoteGenerator (default 1.0). When _set_generator_tempos updates a generator's rhythm streams, it applies new_bpm * generator.tempo_ratio instead of new_bpm directly.

Example usage:

parent = Line().with_rhythm('e').with_pitches('c4 d e f')
child = Line().with_rhythm('q').with_pitches('a2 b')
child.tempo_ratio = 0.5  # half the Link tempo

parent.add_generator(child)
t = kickoff(parent, 'sine.orc', link_follower=lf, streaming=True)
# Link at 120 bpm → parent streams at 120, child streams at 60
# Link changes to 100 → parent at 100, child at 50

Implementation notes

• tempo_ratio on NoteGenerator, default 1.0 (no change to existing behavior)
• _set_generator_tempos changes from stream.tempo = bpm to stream.tempo = bpm * generator.tempo_ratio
• Ratios compose: if parent has ratio 1.0 and child has ratio 0.5, child gets bpm * 0.5. If a grandchild has ratio 2.0, it gets bpm * 0.5 * 2.0 = bpm * 1.0. Whether ratios should compose or be absolute is a design choice — composing feels more natural for hierarchical structures.
• No impact on latency_offset_secs or sync model — those operate in wall time, not beat time