Add framed parsing, validation, benchmarks, and fuzzing

[RejectKid]

Summary

• make permissive non-v7 parsing preserve bodies as UnknownPdu and replace stale release-candidate examples with version-neutral guidance
• add header-only inspection and framed readers for contiguous spans and segmented ReadOnlySequence<byte> buffers
• add builders for Entity State, Fire, Detonation, and Transmitter PDUs plus structured, non-mutating semantic validation
• expand BenchmarkDotNet coverage across representative, variable, vendor-defined, framed, header-only, and malformed inputs
• add a SharpFuzz target, deterministic smoke corpus, scheduled CI workflow, contributor guidance, and security documentation

Validation

• Release build: .NET 8, .NET 9, and .NET 10
• Unit tests: 320 passed on each supported runtime
• DIS schema verification: 233 classes and all 72 standardized PDU types
• Benchmark dry-run matrix: 102 cases passed across .NET 8/9/10
• Fuzz smoke corpus: passed
• NuGet package validation: passed
• Packed-package consumer smoke tests: passed on .NET 8/9/10
• dotnet format --verify-no-changes --no-restore
• git diff --check