Add JAX triangular vector helpers

[FlorianPfaff]

Summary

• expose JAX backend triangular-vector helper support
• add backend contract coverage for triangular vector helpers

Validation

• git apply --recount --check --index C:\Users\emper\Downloads\patches\pyrecest-jax-triangular-vector-helpers.patch
• git diff --cached --check
• conflict-marker scan with rg

Per request, I did not run tests.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	22.39s
✅ PYTHON	black	2	0	0	0	2.45s
✅ PYTHON	isort	2	0	0	0	0.38s
✅ REPOSITORY	checkov	yes		no	no	28.52s
✅ REPOSITORY	gitleaks	yes		no	no	9.7s
✅ REPOSITORY	git_diff	yes		no	no	0.04s
✅ REPOSITORY	secretlint	yes		no	no	9.56s
✅ REPOSITORY	syft	yes		no	no	1.91s
✅ REPOSITORY	trivy-sbom	yes		no	no	3.16s
✅ REPOSITORY	trufflehog	yes		no	no	13.66s

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