Hide spinner Easter eggs from help

[Widthdom]

CLOSES #219

What

• Hide the seven themed spinner flags and --random-spinner from normal nildiff --help output.
• Remove their advertising from current English/Japanese user documentation and the annotated configuration sample.
• Keep all eight flags parseable for backward compatibility.
• Add regression coverage for help output and current user-facing documentation while preserving historical CHANGELOG entries.

Why

Spinner themes are Easter eggs and should remain discoverable only by knowing the flags, without removing existing behavior. Per the follow-up scope decision, --random-spinner is hidden alongside the seven named themes.

Impact

Normal options such as --creator, --open-reports, --open-logs, and --bell remain documented and functional. Existing invocations such as nildiff oldfolder newfolder reportlabel --creator --no-il-cache --coffee remain accepted.

Validation

• dotnet format FolderDiffIL4DotNet.sln --verify-no-changes --no-restore --verbosity minimal
• dotnet build FolderDiffIL4DotNet.sln --configuration Release --no-restore --nologo -m:1 — 0 warnings, 0 errors
• Focused help/documentation and spinner-parser tests — passed
• Full Release tests — 2038 passed, 1 expected E2E skip, 0 failed
• codex review --base origin/main — no actionable findings

Checklist

• English and Japanese documentation kept aligned
• Historical CHANGELOG references preserved
• No tags created or packages published