Add self-validation test descriptions to README and user guide

[Copilot]

The README Self Validation section showed example output listing test names but gave no indication of what each test actually validates, leaving readers to infer purpose from names alone. The user guide lacked any description of the individual self-validation tests.

Pull Request

Description

• README.md — Added a concise one-line bullet-list after the example output block enumerating all 14 self-validation tests, each with a short description of the command exercised. Bullets omit the redundant "Proves the" prefix (the intro sentence establishes context) and use "SPDX file" consistently. Added a sentence linking to the user guide for more details.
• docs/guide/guide.md — Added a new ## Validation Tests section with detailed multi-line descriptions for each of the 14 self-validation tests, explaining what inputs are used, which command is exercised, and what is verified in the output.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code quality improvement

Related Issues

Pre-Submission Checklist

Before submitting this pull request, ensure you have completed the following:

Build and Test

• Code builds successfully: dotnet build --configuration Release
• All unit tests pass: dotnet test --configuration Release
• Self-validation tests pass:
  dotnet run --project src/DemaConsulting.SpdxTool --configuration Release --framework net10.0
--no-build -- --validate
• Code produces zero warnings

Code Quality

• Code formatting is correct: dotnet format --verify-no-changes
• New code has appropriate XML documentation comments
• Static analyzer warnings have been addressed

Quality Checks

Please run the following checks before submitting:

• Spell checker passes: cspell "**/*.{md,cs}"
• Markdown linter passes: markdownlint "**/*.md"
• YAML linter passes: yamllint .

Testing

• Added unit tests for new functionality
• Updated existing tests if behavior changed
• All tests follow the AAA (Arrange, Act, Assert) pattern
• Test coverage is maintained or improved

Documentation

• Updated README.md (if applicable)
• Updated ARCHITECTURE.md (if applicable)
• Added code examples for new features (if applicable)
• Updated requirements.yaml (if applicable)

Additional Notes

Documentation-only change. No code, tests, or build configuration modified.

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[Malcolmnixon]

@copilot Please shorten the Self-validation bullet-list descriptions so they fit on one line.

[Copilot]

@copilot Please shorten the Self-validation bullet-list descriptions so they fit on one line.

Done in 8bdcba3 — all descriptions are now on one line. The longer descriptions were moved to a new "Validation Tests" section in the user guide (86fe8ab), and the README now links to the user guide for more details.

[Malcolmnixon]

Seeing as the heading above the bullet list talks about "proves", we can drop "Proves the" from each bullet.
Let's replace "SPDX document" with "SPDX file" in all of these.

[Copilot]

Done in f68ffa2 — dropped "Proves the" from all bullets and replaced "SPDX document" with "SPDX file" throughout.