instrument: latch the note done at the read-head run-off, fit the migrated fades, and lift the curve dial and overlay selection into pure modules

src/core/instrument/map/component_state_io.h

@@ -91,8 +91,26 @@ namespace reasampler::instrument::map {
 // seconds at the project rate the reader is handed. v10 writes ZERO into both — the values
 // live in the AHD now, so a DOWNGRADE to a pre-v10 binary loses the Trigger amp shape.
 //
+// LOSSY UNDER A RATE MISMATCH. The fades were SOURCE frames and the AHD stores wall-clock
+// seconds, so the lift divides by the PROJECT rate while the build later multiplies by the
+// DECODE rate: a file whose own rate differs from the project's comes back scaled by that
+// ratio (a 441-frame fade on a 44.1 kHz file in a 48 kHz project resolves to 405 source
+// frames, ~8% short). The codec cannot close this — it never sees the file — and deferring the
+// lift to build time would mean carrying the retired fade pair through the parameter set,
+// reintroducing the mechanism the AHD replaced.
+//
 // A truncated/unknown/empty payload yields the DEFAULT parameter set.
 
+// The exponents the lifted fades take. The AHD's curve law is phi^p (core/util/curve_law.h),
+// which cannot reproduce the retired pair's equal-power sin/cos exactly at ANY exponent — so
+// the lift takes the MINIMAX fit rather than the linear neutral, which is free (one constant,
+// written once here) and several times closer. The two differ because the two stages fit
+// different forms: attack fits phi^p to sin(pi*phi/2), decay fits 1 - t^q to cos(pi*t/2).
+// The measured bounds are asserted in tests/test_component_state_io.cpp. Every OTHER curve on
+// a migrated blob still lifts to the neutral — only the fades had a prior shape to reproduce.
+inline constexpr double kTriggerFadeLiftAttackCurve = 0.6133;
+inline constexpr double kTriggerFadeLiftDecayCurve  = 1.7437;
+
 inline constexpr std::uint32_t kPerformanceStateVersion = 2;
 
 // The params-payload format version and its detection marker. The marker is a high sentinel

src/core/instrument/map/params_payload.cpp

@@ -61,15 +61,17 @@ void putAhd(std::vector<std::uint8_t>& out, const AhdSeconds& a) {
 }
 // THE lift of the retired Trigger fade pair onto the AHD that replaced it: Attack takes the
 // fade-in, Decay the fade-out, Hold the whole remainder — so a zero fade-out lands Decay = 0
-// and the abrupt end an old instance could express stays representable. The fades were SOURCE
-// frames and the AHD stores wall-clock seconds, so the conversion goes through the same
-// project rate the v3 lift already uses. A v10-or-newer blob overwrites this from its own tail.
+// and the abrupt end an old instance could express stays representable. The seconds conversion
+// and its rate-mismatch bound, and the two fitted exponents, are documented in
+// component_state_io.h. A v10-or-newer blob overwrites all five fields from its own tail.
 void liftTriggerFades(std::int64_t fadeInFrames, std::int64_t fadeOutFrames, double projectRate,
                       AhdSeconds& out) {
     const double rate = projectRate > 0.0 ? projectRate : 1.0;
     out.attackSeconds = static_cast<double>(fadeInFrames > 0 ? fadeInFrames : 0) / rate;
     out.decaySeconds  = static_cast<double>(fadeOutFrames > 0 ? fadeOutFrames : 0) / rate;
     out.holdFraction  = 1.0;
+    out.attackCurve   = kTriggerFadeLiftAttackCurve;
+    out.decayCurve    = kTriggerFadeLiftDecayCurve;
 }
 
 // Read the play tail (v5 shape onward) into `p`. Shared by the legacy zone reader and the