Preserve signed ellipse axes in extent conversion

[FlorianPfaff]

Summary

• convert ellipse semi-axis coordinates to magnitudes before constructing the extent matrix
• preserve the physical extent for equivalent negative-axis representations
• add focused regression coverage for both the direct and shape-state conversion paths

Bug

ellipse_extent_matrix() clipped negative semi-axis coordinates to zero before squaring them. Elsewhere in the ellipse geometry API, axis signs are treated as representation choices and canonicalized with an absolute value. As a result, a valid signed shape state such as [theta, -3, -1] produced a collapsed extent matrix instead of the same extent as [theta, 3, 1].

Fix

Use the active backend's absolute-value operation before squaring the semi-axis coordinates. This keeps NumPy, PyTorch, and JAX backend behavior aligned with the module's existing canonicalization semantics.

Validation

• reproduced the failure on the main implementation with NumPy-backed module stubs
• verified sign invariance for each single-axis and double-axis sign flip after the change
• python -m py_compile src/pyrecest/tracking/ellipse_geometry.py tests/tracking/test_ellipse_extent_negative_axes.py

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
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✅ JSON	v8r	7		0	0	4.66s
✅ MARKDOWN	markdownlint	68	0	0	0	1.92s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.61s
✅ PYTHON	black	1490	182	0	0	101.73s
✅ PYTHON	isort	1490	328	0	0	2.72s
✅ REPOSITORY	betterleaks	yes		no	no	2.69s
✅ REPOSITORY	checkov	yes		no	no	59.27s
✅ REPOSITORY	gitleaks	yes		no	no	15.63s
✅ REPOSITORY	git_diff	yes		no	no	0.31s
✅ REPOSITORY	secretlint	yes		no	no	72.78s
✅ REPOSITORY	syft	yes		no	no	6.79s
✅ REPOSITORY	trivy-sbom	yes		no	no	7.07s
✅ REPOSITORY	trufflehog	yes		no	no	34.01s
✅ YAML	prettier	11	0	0	0	0.79s
✅ YAML	v8r	11		0	0	11.09s
✅ YAML	yamllint	11		0	0	0.64s

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