Reject out-of-range index-matrix fill values

[FlorianPfaff]

Bug

tracks_to_index_matrix() validates that fill_value is a negative integer, but did not validate that it is representable by the integer matrix it allocates. A value below numpy.iinfo(numpy.int64).min therefore passed the public validation and failed later inside the backend allocator with a raw OverflowError.

For example, fill_value=numpy.iinfo(numpy.int64).min - 1 reached numpy.full(..., dtype=int) and raised OverflowError: Python int too large to convert to C long.

Fix

• reject negative fill values below the int64 range with a deterministic ValueError;
• preserve the valid int64 minimum boundary;
• add regression coverage for both the immediate underflow case and a much larger negative Python integer.

Validation

• reproduced the pre-fix OverflowError through the current module in an isolated NumPy-backend harness;
• the patched focused test module passes all three index-matrix tests;
• syntax compilation passes for both modified files;
• branch comparison is limited to two files: 3 source lines and 23 regression-test lines.

The repository's pull-request workflows provide the full supported Python/backend, lint, packaging, documentation, and security validation matrix.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	27.81s
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✅ JSON	v8r	7		0	0	4.76s
✅ MARKDOWN	markdownlint	68	0	0	0	1.95s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.81s
✅ PYTHON	black	1809	56	0	0	94.96s
✅ PYTHON	isort	1809	88	0	0	2.88s
✅ REPOSITORY	betterleaks	yes		no	no	2.39s
✅ REPOSITORY	checkov	yes		no	no	56.86s
✅ REPOSITORY	gitleaks	yes		no	no	17.37s
✅ REPOSITORY	git_diff	yes		no	no	0.45s
✅ REPOSITORY	secretlint	yes		no	no	78.91s
✅ REPOSITORY	syft	yes		no	no	6.07s
✅ REPOSITORY	trivy-sbom	yes		no	no	6.34s
✅ REPOSITORY	trufflehog	yes		no	no	33.59s
✅ YAML	prettier	11	0	0	0	0.87s
✅ YAML	v8r	11		0	0	12.37s
✅ YAML	yamllint	11		0	0	0.57s

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[FlorianPfaff]

Closing as low value. This only changes the exception produced for an already-invalid integer sentinel outside the representable int64 range. Supported sentinel values are unaffected, so the maintenance and test surface is not justified by practical user impact.