fix(instrument-ui): distinguish Disabled from Off, add hover cue to mode toggles, close review minors

Adds a hairline outline for Disabled chrome buttons, resolves Hover on the four
mode-selector single-buttons, re-measures the control-surface doc's 1.2 layout
table post-reflow, and adds a structural test pinning every enable/mode toggle
to a derived segment.

src/core/instrument/ui/deck_groups.cpp

@@ -18,8 +18,9 @@ double clamp(double v, double lo, double hi) { return v < lo ? lo : (v > hi ? hi
 constexpr int kEnvModeW = 46;
 
 // The single-button enables ("Envelope" / "Filter" / "Limiter"). One width because they are one
-// control grammar. MASTER is the group that BINDS it: its caption row and knob row both measure
-// 130, so anything past 64 widens kDeckSpanningW and spends the editor's width budget.
+// control grammar. MASTER is the group that BINDS it: its knob row measures 130 (the caption
+// row, at this width, measures 118), so anything past 64 widens kDeckSpanningW and spends the
+// editor's width budget.
 constexpr int kEnableBtnW = 52;
 
 DeckToggleDesc enableButton(DeckParam p) {

src/core/instrument/ui/sample_chrome.cpp

@@ -18,7 +18,7 @@ constexpr int kStripBandHeight = 30;
 
 constexpr int kRunGap = 6;  // between adjacent items of the toolbar run
 constexpr int kChanBtnW = 60;   // fits the longer of the two mode labels ("Stereo")
-constexpr int kChanSegH = 18;
+constexpr int kChanBtnH = 18;   // shared height of the Loop and channel-mode single buttons
 // If the title slot ever fails to hold its text at the editor's floor, THESE narrow — the
 // floor does not move.
 constexpr int kLoopBtnW = 52;
@@ -54,14 +54,14 @@ ChromeRects chromeRects(const Rect& chrome, int knobSize) {
     r.navBrowse = Rect::ltrb(leftOf(navRight, kNavButtonWidth), navTop, navRight,
                              navTop + navH);
 
-    const int chanTop = topFor(kChanSegH);
+    const int chanTop = topFor(kChanBtnH);
     const int chanRight = leftOf(r.navBrowse.x, kRunGap);
     r.channel = Rect::ltrb(leftOf(chanRight, kChanBtnW), chanTop, chanRight,
-                           chanTop + kChanSegH);
+                           chanTop + kChanBtnH);
 
     const int loopRight = leftOf(r.channel.x, kRunGap);
     r.loop = Rect::ltrb(leftOf(loopRight, kLoopBtnW), chanTop, loopRight,
-                        chanTop + kChanSegH);
+                        chanTop + kChanBtnH);
 
     const int cellH = std::min(row.height, knobSize + kVelLabelH);
     const int cellTop = topFor(cellH);