instrument: one staged-envelope system — per-segment curves, the sustain-less AHD, and a shared overlay for all three envelopes

Trigger's fade pair folds into the AHD (and goes live); the release anchors right;
Preserve rings its synthetic tail out instead of cutting it. Payload v10.

src/core/instrument/ui/knob_deck.h

@@ -35,6 +35,11 @@ inline constexpr int kDeckCaptionGap = 2;     // caption row -> knob row gap
 inline constexpr int kDeckToggleGap = 4;      // caption text -> toggle / cells -> row toggle gap
 inline constexpr int kDeckGroupGap = 12;      // gap between groups on a row
 inline constexpr int kDeckRowGap = 8;         // gap between wrapped deck rows
+inline constexpr int kDeckRadioSize = 12;     // the caption-row corner radio square
+// The knob cell's INNER dial: a concentric sub-disc that edits a second, related value while
+// the outer ring keeps editing the cell's own. Geometry only — WHICH cells carry one is
+// deck_groups' call, so a cell without an inner value simply resolves an inner hit as a knob.
+inline constexpr int kDeckInnerDialSize = 14;
 // One group box: padding + caption + gap + cell row + padding.
 inline constexpr int kDeckGroupH =
     kDeckGroupPadY + kDeckCaptionH + kDeckCaptionGap + kDeckCellH + kDeckGroupPadY;
@@ -45,13 +50,20 @@ struct DeckToggleDesc {
     int segWidth = 44;  // px per segment
 };
 
+// A single-square corner radio (an exclusive selector across groups, so the group itself
+// carries no state). id -1 = absent.
+struct DeckRadioDesc {
+    int id = -1;
+};
+
 // One fenced group, in deck order. `cellIds` are the knob cells left-to-right; an id of -1
 // is a reserved blank cell (geometry held, never hit). `captionWidth` is the px the shell
 // reserves for the caption text (this module does not measure text).
 struct DeckGroupDesc {
     int id = 0;                    // shell group id (opaque here)
     int captionWidth = 60;
-    DeckToggleDesc captionToggle;  // right-anchored in the caption row; id -1 = none
+    DeckRadioDesc captionRadio;    // the caption row's far corner; id -1 = none
+    DeckToggleDesc captionToggle;  // caption row, left of the radio; id -1 = none
     std::vector<int> cellIds;      // knob cells; -1 = blank reserve
     DeckToggleDesc rowToggle;      // in the knob row after the cells; id -1 = none
 };
@@ -64,10 +76,16 @@ struct DeckToggleLayout {
     Rect seg1;  // right segment
 };
 
+struct DeckRadioLayout {
+    int id = -1;
+    Rect box;
+};
+
 struct DeckCellLayout {
     int id = -1;
     Rect cell;   // the full 48x58 cell
     Rect knob;   // the centered kDeckKnobSize square (the knob circle inscribes it)
+    Rect inner;  // the concentric kDeckInnerDialSize square inside `knob`
     Rect label;  // the 12px label band beneath the knob
 };
 
@@ -75,6 +93,7 @@ struct DeckGroupLayout {
     int id = 0;
     Rect box;                       // the fenced group box
     Rect caption;                   // caption text rect (left part of the caption row)
+    DeckRadioLayout captionRadio;   // id -1 when absent (rect empty)
     DeckToggleLayout captionToggle; // id -1 when absent (rects empty)
     std::vector<DeckCellLayout> cells;
     DeckToggleLayout rowToggle;     // id -1 when absent
@@ -105,17 +124,19 @@ DeckLayout layoutDeck(const std::vector<DeckGroupDesc>& groups, int left, int to
 
 // --- Hit-test --------------------------------------------------------------------------
 
-enum class DeckHitKind { None, Knob, CaptionToggle, RowToggle };
+enum class DeckHitKind { None, Knob, CaptionToggle, RowToggle, CaptionRadio };
 
 struct DeckHit {
     DeckHitKind kind = DeckHitKind::None;
-    int id = -1;       // the control id of the hit element (cell id / toggle id)
+    int id = -1;       // the control id of the hit element (cell id / toggle id / radio id)
     int segment = -1;  // 0/1 for a toggle hit; -1 otherwise
+    bool inner = false;  // Knob hits only: the grab landed on the cell's inner dial
 };
 
 // The deck element a point lands on: a knob cell (the whole cell, not just the knob
-// circle — the shell anchors the vertical drag wherever the grab lands), a caption-toggle
-// segment, or a row-toggle segment. Blank cells (id -1) and everything else miss.
+// circle — the shell anchors the vertical drag wherever the grab lands, with `inner` marking
+// a grab on the concentric inner dial), a caption-toggle segment, a row-toggle segment, or
+// the caption-row corner radio. Blank cells (id -1) and everything else miss.
 DeckHit hitTestDeck(const DeckLayout& layout, int x, int y);
 
 }  // namespace reasampler::instrument::ui