Draw grid lines and labels for inverted axes with INCREMENT_BY_VAL

[halfhp]

Fixes #125

Cause

With inverted boundaries, e.g. setDomainBoundaries(10, 0, FIXED), the axis region has min greater than max, so Region.ratio is negative. In XYStepCalculator, the INCREMENT_BY_VAL and INCREMENT_BY_FIT modes derive the pixel step by dividing the value step by that ratio, giving a negative pixel step. XYGraphWidget.drawGrid walks each axis in pixel order and computes its loop bounds from the pixel step, so with a negative step the bounds came out reversed and the loop never ran: no grid lines, no labels. The other modes derive the pixel step from pixel lengths, so they stayed positive and worked, which is why the reporter only saw it with INCREMENT_BY_VAL.

Fix

In XYStepCalculator, when the computed pixel step is negative, negate both it and the value step. The pixel step is then always positive and the value step carries the axis direction, which is exactly what INCREMENT_BY_PIXELS and SUBDIVIDE already produce for inverted bounds. drawGrid needs no changes: it already computes each line's value as origin plus index times the signed value step.

This is a narrower change than the reporter's proposed swap of the loop endpoints inside drawGrid, and it fixes the inconsistency at its source so any other consumer of Step sees the same convention across modes.

Verification

• StepCalculatorTest: new case with inverted real bounds asserts INCREMENT_BY_VAL yields pixel step +1 and value step -1, and that INCREMENT_BY_PIXELS and SUBDIVIDE produce the same signs. Fails on the old code with pixel step -1.
• XYGraphWidgetTest: new case sets inverted domain and range bounds with INCREMENT_BY_VAL and captures every grid line drawn. Asserts 101 lines per axis, that the leftmost domain line carries the value 100 and the rightmost 0, and that the top range line carries 0 and the bottom 100. Fails on the old code with zero lines drawn.
• Full suite: 218 tests, 0 failures. Lint, core release AAR and demoapp debug build pass.
• Release note added under 1.5.12.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.02%. Comparing base (c15129b) to head (bfd8592).
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Additional details and impacted files

@@             Coverage Diff              @@
##             master     #137      +/-   ##
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+ Coverage     70.98%   71.02%   +0.03%     
- Complexity     1242     1244       +2     
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  Files           109      109              
  Lines          5191     5194       +3     
  Branches        543      544       +1     
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+ Hits           3685     3689       +4     
+ Misses         1151     1150       -1     
  Partials        355      355              

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