Add comprehensive test coverage for BoundingBox, PointInImage, SNTColor, LinAlgUtils, StrahlerAnalyzer, and PathStatistics

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Summary

Adds 6 new test files targeting areas of the codebase with no existing tests. Before this PR there were 20 test files covering 369 source files (~5.4% test-to-source ratio); this PR adds 6 new test classes with ~200 individual test cases.

New Test Files

sc.fiji.snt.util.PointInImageTest

Covers the core geometric point class:

• Distance calculations (Euclidean, squared Euclidean, Chebyshev XY/Z/3D)
• isReal() for NaN/Infinite coordinate detection
• isSameLocation() comparison
• scale() coordinate transformation
• getCoordinateOnAxis() with valid/invalid axis values
• equals(), hashCode(), clone() contract

sc.fiji.snt.util.BoundingBoxTest

Covers the bounding box utility:

• Default and point-cloud constructors
• compute(), append(), combine(), intersection()
• contains(BoundingBox), contains(SNTPoint), contains2D(SNTPoint)
• getDimensions(), getDiagonal(), getCentroid()
• scale(), shift() spatial transformations
• isScaled(), hasDimensions(), setDimensions() (including error case)
• sanitizedUnit() for various string inputs (null, "pixels", "um", "micron", custom)
• equals(), hashCode(), clone()

sc.fiji.snt.util.SNTColorTest

Covers the color utility class:

• colorToString() for null, AWT Color, and ColorRGB inputs
• fromHex() for 6-digit, 8-digit, hash-prefixed, and named color strings
• fromString() consistency with fromHex()
• average() for empty, single, two-color, and null-containing collections
• interpolateNullEntries() for middle nulls, null array, and all-null cases
• alphaColor() for 0%, 50%, 100% and null input
• contrastColor() for dark and light inputs
• getDistinctColors(), getDistinctColorsHex(), getDistinctColorsAWT() with various counts and excluded hues

sc.fiji.snt.util.LinAlgUtilsTest

Covers the linear algebra utility:

• Reflection across XY, XZ, and YZ planes
• Points on the plane are unchanged by reflection
• Involutory property: reflecting twice returns the original point
• Matrix dimensions (4×4 homogeneous) and last-row correctness

sc.fiji.snt.analysis.StrahlerAnalyzerTest

Covers Horton-Strahler analysis on the demo fractal tree:

• getRootNumber() correctness and consistency with TreeStatistics.getStrahlerNumber()
• getHighestBranchOrder() bounds
• getBranchCounts(), getLengths(), getBranchPointCounts() map structure and value validity
• getBifurcationRatios() — NaN for highest order, positive for lower orders, avg = 2 for fractal tree
• getBranches() for all valid orders; IllegalArgumentException for out-of-range orders
• getAvgFragmentations(), getAvgContractions(), getGraph(), getRootAssociatedBranches()

sc.fiji.snt.analysis.PathStatisticsTest

Covers path-level morphometric analysis:

• Single-path and multi-path construction
• getNBranches(), getBranches() count
• getMetric(String) for Path ID (single vs. multi path)
• getMetric(String, Path) for length, node count, and children count
• getPrimaryBranches(), getTerminalBranches(), getInnerBranches()
• getPrimaryLength(), getTerminalLength(), getInnerLength()
• getCableLength() consistency with individual path lengths
• Manual path with known geometry (3-4-0 triangle → length 5)