test: cover shaded-region patch length-parity fix (codecov patch coverage)

Add two regression tests to TestAddShaded exercising all three code paths the
shading fix changed:
- testShadedPatchEqualLengthCacheBranch: n>20000 hits render()'s cache branch,
  then a zoom exercises updateShadings().
- testShadedPatchEqualLengthMidBranch: 5000<n<=20000 hits render()'s mid-size
  branch.

A monotonic upper curve + constant fill baseline deterministically makes the
two boundaries downsample to different lengths (minmax tail-anchor), so each
test would have thrown "Vectors must be the same length." on the pre-fix code
and now asserts equal-length patch XData/YData. Covers the 15 lines flagged by
codecov/patch. Lives in TestAddShaded (A-D shard) to stay clear of the flaky
TestTagPerfRegression perf gate in the Q-Z shard.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: HanSur94

tests/suite/TestAddShaded.m

@@ -101,5 +101,55 @@ function testAddFillRendered(testCase)
             ud = get(fp.Shadings(1).hPatch, 'UserData');
             testCase.verifyEqual(ud.FastSense.Type, 'shaded', 'testAddFillRendered: type is shaded');
         end
+
+        function testShadedPatchEqualLengthCacheBranch(testCase)
+            % Regression (260512-c5x): with > 2*shadingCacheSize (20000) points
+            % and a constant fill baseline, the upper (varying) and lower
+            % (constant) boundaries downsample to DIFFERENT lengths via the
+            % minmax tail-anchor. render()'s cache branch must still emit an
+            % equal-length patch polygon — previously patch() threw "Vectors
+            % must be the same length." (e.g. example_dock's Power Systems
+            % tile). The subsequent zoom exercises the same fix in
+            % updateShadings(). A monotonic upper curve guarantees divergence:
+            % its last sample IS its last bucket extreme (no tail-anchor) while
+            % the constant baseline DOES anchor.
+            n = 25000;
+            x = linspace(0, 100, n);
+            y = linspace(0, 10, n);
+            fp = FastSense();
+            fp.addLine(x, y, 'DisplayName', 'ramp');
+            fp.addFill(x, y, 'Baseline', 0, 'FaceColor', [0 0.5 1]);
+            fp.render();
+            testCase.addTeardown(@close, fp.hFigure);
+
+            hP = fp.Shadings(1).hPatch;
+            testCase.verifyEqual(numel(get(hP, 'XData')), numel(get(hP, 'YData')), ...
+                'cache branch: shading patch XData/YData length mismatch after render');
+
+            % Zoom to a sub-range that stays above MinPointsForDownsample so
+            % updateShadings re-downsamples (the live zoom/pan path).
+            set(fp.hAxes, 'XLim', [20 60]);
+            drawnow; pause(0.2);
+            testCase.verifyEqual(numel(get(hP, 'XData')), numel(get(hP, 'YData')), ...
+                'updateShadings: shading patch XData/YData length mismatch after zoom');
+        end
+
+        function testShadedPatchEqualLengthMidBranch(testCase)
+            % MinPointsForDownsample (5000) < n <= 2*shadingCacheSize (20000)
+            % exercises render()'s mid-size (elseif) downsample branch with the
+            % same varying-vs-constant boundary divergence.
+            n = 10000;
+            x = linspace(0, 100, n);
+            y = linspace(0, 5, n);
+            fp = FastSense();
+            fp.addLine(x, y);
+            fp.addFill(x, y, 'Baseline', 0);
+            fp.render();
+            testCase.addTeardown(@close, fp.hFigure);
+
+            hP = fp.Shadings(1).hPatch;
+            testCase.verifyEqual(numel(get(hP, 'XData')), numel(get(hP, 'YData')), ...
+                'mid branch: shading patch XData/YData length mismatch after render');
+        end
     end
 end