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.

tests/test_deck_groups_measured.cpp

@@ -422,8 +422,8 @@ static void testEnvModeCeilingsArePinnedForPitchEnvAndAmp() {
 
 // Every group's width, in BOTH play modes, against the measured layout table
 // (instrument-control-surface.md §1.2). Mode-independence is the second half of the claim: the
-// reserve slots hold the two mode-dependent groups at 312 either way, which is what makes the
-// contour row's 876 a constant rather than a Gate-only fact.
+// reserve slots hold the two mode-dependent groups at 372 (FILTER ENV) and 312 (AMP ENVELOPE)
+// either way, which is what makes the contour row's 936 a constant rather than a Gate-only fact.
 static void testEveryGroupWidthMatchesTheMeasuredLayout() {
     const struct { int id; int width; } want[] = {
         {kGroupPitch, 192}, {kGroupPitchEnv, 252}, {kGroupFilter, 372},
@@ -477,6 +477,7 @@ static void testTheGateFaceIsPixelIdenticalApartFromTheTwoFilterGroups() {
         {kGroupPitchEnv,   0, 252, 4},  // unchanged
         {kGroupFilterEnv, 298, 372, 6}, // was x=328 w=312 with 5 cells
         {kGroupAmpEnv,   716, 312, 5},  // unchanged
+        {kGroupMaster,  1040, 142, 1},  // unchanged — the claim above actually pins it
     };
     for (const auto& w : want) {
         const DeckGroupLayout& l = lay(w.id);