insert_tremolo: the missing surf/exotica amplitude modulation

The effects audit found two defining effects absent for infrastructure reasons: surf rock's Fender-amp tremolo and exotica's vibraphone fan. One module serves both

• Probe-verified: bypass bit-identical (maxAbsDiff 0); envelope peak-to-peak 0.7000 at depth 0.7, DFT locked to the LFO rate (14x over neighbors)

• NOT yet wired: the apply round adds the insertChain case, manifest entry, prewarm-list entry, and the surfrock/exotica recipes (spec in the round report).

▶ Play it: Undertow Sock Hop — parked on the genre this change names https://stellate.app/?genre=surfrock&seed=5991&path=1315.6181,1334.6214,1296.6214&xdur=13.122

160b9d9 https://github.com/aboard-io/stellate/commit/160b9d97a2ab355fd028f991d1dec149cc4ac124