fix: close review findings on spline EGs — engine, codec, and popup/overlay UI grammar

Live pitch depth, Gate/Spline enable-rule agreement, inert kTrigLength, NaN wire guards, hard-flag-tail corruption no longer wipes the record, RT/cold spline tie-break, retired alt-click, marker-shadow fix, plus new test coverage.

src/shell/instrument/reasampler_editor.h

@@ -216,8 +216,7 @@ private:
 
     // Mouse-down inside curve-editor box `r`, resolved through the shared point-editing
     // grammar (spline_edit): a node grab starts a kCurveNode drag, an empty-space click adds a
-    // point and grabs it, control-click toggles a node hard/smooth. Alt-click delete predates
-    // that grammar and is kept as a landed alternate.
+    // point and grabs it, control-click toggles a node hard/smooth.
     void handleCurveMouseDown(const Rect& r, int x, int y);
 
     // Left-click while the curve popup is open (modal over the Sample face): Close /
@@ -270,8 +269,8 @@ private:
     void commitLive();
 
     // Whether an in-flight drag commits live rather than through a reload. A deck knob is
-    // live per isLiveDeckParam; an envelope-node drag is live only in Gate, where it edits the
-    // AHDSR — in Trigger the same drag rewrites the play span, which is not a live control.
+    // live per isLiveDeckParam; an envelope-node drag is live in EITHER mode — see
+    // liveCommitFor (deck_groups.h) for why.
     bool dragCommitsLive(DragKind kind, int paramId = -1) const;
 
     // Commits `id` as the loaded capture. The one parameter set carries over — it governs