Epic: Modernize string-similarity contract coverage and compatibility proof

Repository: lancer1977/StringSimilarity.NET (GitHub Issues are disabled).

Goal

Make algorithm semantics, Unicode behavior, and package compatibility reliably testable for current consumers.

Recommendations

1. Document algorithm selection, metric/similarity guarantees, normalization ranges, null/empty behavior, and Unicode/culture assumptions.
2. Add property-style tests for symmetry, bounds, identity, metric properties where promised, Unicode/grapheme cases, and large-input limits.
3. Reconcile the netstandard2.0 library with its net6.0 test target and document the supported compatibility matrix.

Acceptance criteria

• Consumers can choose an algorithm from documented guarantees and examples.
• Tests protect core invariants and non-ASCII edge cases.
• Compatibility and validation targets are documented and continuously verified.

Suggested child slices

• Publish algorithm decision/contract guide.
• Add invariant, Unicode, and large-input tests.
• Verify and document framework compatibility.