Migrate scoring APIs to double precision for v6.0.0

[ycherkes]

Closes #28

Summary

• migrate scorer interfaces, cached scorers, scoring strategies, Fuzz methods, extraction results, and extraction cutoffs from int to double
• preserve fractional similarity values throughout standard, cached, parallel, and pipeline extraction paths
• remove scoreCutoff from the Indel, Levenshtein, longest-common-subsequence, and partial-ratio APIs
• retain partial-ratio pruning as an internal implementation detail while removing cutoff behavior from the public scoring API
• optimize character similarity and token-set scoring hot paths by specializing common-affix trimming, bulk-populating pattern-match vectors, and reducing temporary allocations
• update tests, benchmarks, examples, and documentation for double-precision scores
• bump package, assembly, and file versions to 6.0.0
• replace the legacy branch-specific workflows with a single read-only workflow that builds, tests, packs, and uploads .nupkg and .snupkg files as GitHub Actions artifacts only

Breaking changes

• scorer and strategy return types are now double
• Fuzz methods now return fractional double scores instead of rounded integers
• ExtractedResult<T>.Score, scorer delegates, and extraction cutoff parameters now use double
• scoreCutoff parameters have been removed from the distance and partial-ratio APIs
• custom implementations of IRatioScorer, ICachedRatioScorer, and their generic variants must update their Score return types to double

Validation

• built the complete solution in Release configuration on all configured target frameworks
• passed 5,126 tests on each of netframework4.6.2, netframework4.7.2, net8.0, and net10.0 (20,504 total test executions)
• built the benchmark project successfully
• reduced the extended construction-inclusive static Levenshtein benchmark from 153.0 ns to 123.4 ns (19.3%) and allocation from 144 B to 80 B; cached matching improved from 77.2 ns to 73.3 ns (5.0%)
• generated Raffinert.FuzzySharp.6.0.0.nupkg and Raffinert.FuzzySharp.6.0.0.snupkg
• verified the workflow contains no NuGet registry push, package-write permission, or publishing credentials