loop: crossfade the Gate sustain seam, and unshadow the loop handles that made loop points look gone

src/core/instrument/map/params_payload.cpp

@@ -319,6 +319,8 @@ void putParamsPayload(std::vector<std::uint8_t>& out, const InstrumentParams& p)
     putLE(out, doubleToBits(f.env.decayCurve));
     putLE(out, doubleToBits(f.env.releaseCurve));
     putAhd(out, f.trigEnv);
+    // v11: the loop crossfade, in SOURCE frames.
+    putLE(out, asU64(p.loopCrossfadeFrames));
 }
 
 // Read whichever payload shape follows: the single-record shape (v8 onward, growing by
@@ -351,6 +353,12 @@ PayloadRead readParamsPayload(ByteReader& r, double projectRate) {
     readCurveTail(r, p.velocityCurve);
     if (pv >= kParamsFilterVersion) readFilterTail(r, p);
     if (pv >= kParamsCurveVersion) readCurveStageTail(r, p);
+    if (pv >= kParamsLoopVersion) {
+        // A negative fade is meaningless and would reach resolveLoop's clamp anyway; refusing
+        // it here keeps the parameter set itself sane for the editor that reads it back.
+        const std::int64_t xf = r.i64();
+        p.loopCrossfadeFrames = xf > 0 ? xf : 0;
+    }
     // A truncated record leaves whatever parsed plus construction defaults for the rest —
     // the same degrade-don't-throw contract the zone ladder always had.
     if (!r.ok) return PayloadRead{};