Fix backend contract edge cases

[FlorianPfaff]

Summary

• make NumPy divide-with-zero handling work with scalars, broadcasting, and integer inputs
• add JAX backend implementations for divide, dot, outer, batched matvec, set_diag, vec_to_diag, and triangular matrix reconstruction edge cases
• extend backend contract coverage for these shape and dtype cases

Validation

• git diff --check
• conflict-marker scan with rg
• python -m py_compile src\pyrecest\_backend\_shared_numpy\__init__.py src\pyrecest\_backend\jax\__init__.py tests\test_backend_contracts.py

Tests were not run per request.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	33.14s
✅ PYTHON	black	3	2	0	0	2.28s
✅ PYTHON	isort	3	2	0	0	0.41s
✅ REPOSITORY	checkov	yes		no	no	30.12s
✅ REPOSITORY	gitleaks	yes		no	no	16.17s
✅ REPOSITORY	git_diff	yes		no	no	0.05s
✅ REPOSITORY	secretlint	yes		no	no	13.4s
✅ REPOSITORY	syft	yes		no	no	2.86s
✅ REPOSITORY	trivy-sbom	yes		no	no	3.63s
✅ REPOSITORY	trufflehog	yes		no	no	23.79s

Notices

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[github-actions]

Test Results

    10 files  ± 0      10 suites  ±0   2h 38m 42s ⏱️ - 1m 1s
 2 143 tests + 8   2 139 ✅ + 8      4 💤 ±0  0 ❌ ±0 
21 861 runs  +80  18 171 ✅ +80  3 690 💤 ±0  0 ❌ ±0 

Results for commit 2ab53f6. ± Comparison against base commit 6dbce88.