Skip unused candidate-pruning penalty escalation

[FlorianPfaff]

Summary

• compute the candidate mask before escalating the replacement cost
• return a copy immediately when every matrix entry is retained
• add a regression for an unpruned matrix containing the largest finite float

Bug

prune_pairwise_cost_matrix() always called _effective_large_cost() before checking whether the candidate mask actually pruned anything. For a retained cost equal to np.finfo(float).max, _effective_large_cost() attempted np.nextafter(max_float, np.inf), obtained infinity, and raised ValueError even though no replacement cost was needed.

This made a valid no-op pruning configuration fail solely because the input occupied the upper end of the floating-point range.

Fix

Build the mask first. When all entries are retained, return an owning copy of the validated costs and skip replacement-cost escalation. The existing behavior is unchanged whenever at least one entry must be replaced.

Validation

• isolated regression reproduced the pre-fix failure and passes with the guard
• verified an actually pruned max_float entry still raises because no larger finite replacement exists
• python -m py_compile passes for the modified module and regression test
• full multi-environment validation is delegated to GitHub Actions

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	25.54s
✅ JSON	prettier	7	0	0	0	0.87s
✅ JSON	v8r	7		0	0	2.94s
✅ MARKDOWN	markdownlint	68	0	0	0	2.13s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.38s
✅ PYTHON	black	1490	182	0	0	96.75s
✅ PYTHON	isort	1490	327	0	0	2.65s
✅ REPOSITORY	betterleaks	yes		no	no	2.52s
✅ REPOSITORY	checkov	yes		no	no	55.68s
✅ REPOSITORY	gitleaks	yes		no	no	15.1s
✅ REPOSITORY	git_diff	yes		no	no	0.23s
✅ REPOSITORY	secretlint	yes		no	no	63.51s
✅ REPOSITORY	syft	yes		no	no	5.56s
✅ REPOSITORY	trivy-sbom	yes		no	no	6.71s
✅ REPOSITORY	trufflehog	yes		no	no	31.82s
✅ YAML	prettier	11	0	0	0	0.66s
✅ YAML	v8r	11		0	0	11.25s
✅ YAML	yamllint	11		0	0	0.63s

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