Add comprehensive unit tests for Sheet class

[Copilot]

The Sheet class (model/sheet.h) had minimal test coverage - only a single instantiation in testTensorOps.cc. This PR adds comprehensive unit tests covering the class's public API.

Changes

• New test file: test/testSheet.cc with 15 test cases covering:

  • Geometric calculations (corners)
  • Hit testing (onResizeHandle, onRavelButton, inItem, clickType, contains)
  • Scrolling and keyboard input (scrollUp, scrollDown, onKeyPress)
  • Data operations (exportAsCSV error handling, setSliceIndicator)
  • Properties and flags (showRavel, showRowSlice, showColSlice, dimensions, positioning)
  • Copy semantics (validates no-op behavior)

• Updated: test/Makefile to include testSheet.o in GTESTOBJS

Not covered

• Private methods (ravelSize, ravelX, ravelY) - should not be tested directly
• Rendering methods (draw, drawResizeHandles) - require Cairo context
• computeValue - requires complex port/tensor setup, appropriate for integration tests

Example test coverage

TEST_F(SheetTest, clickType)
{
  moveTo(100, 100);
  iWidth(200);
  iHeight(150);
  double z = zoomFactor();
  double w = 0.5 * 200 * z;
  double h = 0.5 * 150 * z;
  
  EXPECT_EQ(ClickType::onResize, clickType(100 + w, 100 + h));
  EXPECT_EQ(ClickType::inItem, clickType(100, 100));
  EXPECT_EQ(ClickType::onItem, clickType(100 + w * 0.9, 100 + h * 0.9));
  EXPECT_EQ(ClickType::outside, clickType(0, 0));
}

Original prompt

Improve code coverage on Sheet class, found in model/sheet.h

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