Warn about group labels differing only in case or spacing

[prami25r]

Summary

Adds a warning when distinct labels within a dimension match after trimming and full Unicode case-folding. For example, M and m remain separate groups with their original counts, but now raise sex: 'M' and 'm' differ only in case or spacing.

• Python and JavaScript produce the same warnings, surfaced through the existing terminal, browser, HTML, and CSV reports.
• Reuses the Unicode trim set and JS case-folding helper without applying numeric or NFKC normalization.
• Updates SPEC section 3 and adds 65 regression cases covering preserved metrics, Unicode, missing values, property-named labels, dimension scoping, and report parity.

Type

• Bug fix

Validation

• Final fast suite: 903 passed, 2 skipped, 27 deselected (slow tests and four pre-existing Windows failures deselected).
• Full suite also run. Seven unrelated Windows path/regex/error-message failures were reproduced on unchanged upstream aa98442. A SPEC wording failure found during that run was corrected; its checks and the final fast suite pass.
• Ruff, em-dash lint, internal-link checks, and Unicode case-fold parity check pass.
• The issue reproducer adds exactly one collision warning; all other structured fields match the original implementation.

Linked issue

Closes #1009

[vercel]

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[github-actions]

Hey @prami25r, your first PR on Fair Code, that's awesome, thank you.

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[github-actions]

@yakew7 @ahmdkaml - new PR touching a path you own, please review.