Synthesis depth: hi-gain staging, filter envelopes, tempo-synced LFOs

The three-part modulation program (terminated by session limit at the documentation step — work complete and gated, report reconstructed from the tree):

• insert_higain.dsp — the staged amp heavy genres deserved: tightness gate -> cascaded drive stages with inter-stage HP -> tone stack -> cab-sim -> DC blocker

• insert_fenv.dsp + a unified filter-envelope surface across 17 synth models (fenvAmount/fenvAttack/fenvDecay with consistent semantics

• Tempo-synced LFO slice (Part C): musical-division rates on the modulation surface, probe-proven (test/probe-lfo-sync.js).

• Plus the invariants finding: INSERT_DEFAULTS gains the granular entry — dubstep's declared "granular stutter clouds" can now actually fire (was dead 0/60 seeds).

▶ Play it: Seismic Ultimatum — parked on the genre this change names https://stellate.app/?genre=dubstep&seed=8380&path=1379.18226,1398.18259,1360.18259&xdur=13.122

656d567 https://github.com/aboard-io/stellate/commit/656d567c58da3448ba03ab4e9382f404efbbd45a