[scripts] Discover both check and checks TUFLOW folders

[Chain-Frost]

Summary

• Recognise both check and checks as TUFLOW result-folder names, regardless of capitalisation.
• Continue using Python 3.14 Path.walk(), considering directory entries only and retaining deterministic sorting.
• Apply the existing scanning, SHA-256 Rich dashboard, cross-folder comparison, manifest, and placeholder logic unchanged to either name.
• Update documentation and add regression tests for nested mixed-case folders, exclusions of files named check/checks, and duplicate grouping across a check/checks pair.

Validation

• Run Windows Python 3.14 Ruff lint/format, strict Pyright and hosted pytest in CI.
• No changes to hash algorithm, content processing, deletion safety or progress UI.

Follow-up to merged #104 and #105.