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/shell/instrument/editor_input_chrome.cpp

@@ -78,8 +78,9 @@ bool ReaSamplerEditor::mouseDownChrome(const FaceLayout& fl, int x, int y) {
     // still offer to reverse. One button, so the click's meaning is "the other state": read the
     // current enable off the markers, which is the same source the button's paint reads.
     if (loopControlsLive() && contains(cr.loop, x, y)) {
+        // setLoopEnabled owns the frames <= 0 guard (empty capture) — no need to repeat it here.
         const auto frames = static_cast<std::int64_t>(monoPcmFor(selectedId_).size());
-        if (frames > 0) setLoopEnabled(!pickedMarkers(frames).hasLoop);
+        setLoopEnabled(!pickedMarkers(frames).hasLoop);
         invalidate();
         return true;
     }