Add full suite of interfaces to SequentialGuid & SequentialSqlGuid structs

[buvinghausen]

Add interface implementations and implicit operators to SequentialGuid / SequentialSqlGuid

Summary

This PR promotes SequentialGuid and SequentialSqlGuid from simple wrapper structs to first-class .NET citizens by implementing the standard BCL interfaces and adding implicit conversion operators. A shared ISequentialGuid<TSelf> interface captures the common contract.

New: ISequentialGuid<TSelf> interface

Defines the shape shared by both struct types, constraining TSelf : struct, ISequentialGuid<TSelf>:

Target	Interfaces
.NET Framework 4.6.2 / .NET Standard 2.0	IFormattable, IComparable, IComparable<TSelf>, IEquatable<TSelf>
.NET 8–9	Adds ISpanFormattable, ISpanParsable<TSelf>
.NET 10+	Adds IUtf8SpanFormattable, IUtf8SpanParsable<TSelf>

A static abstract Create(Guid) factory method (available on .NET 7+) allows generic code to construct either type from a raw Guid.

Interface implementations on both structs

Interface	Methods
IComparable	CompareTo(object?)
IComparable<TSelf>	CompareTo(TSelf)
IEquatable<TSelf>	Equals(TSelf) — plus GetHashCode() override
IFormattable	ToString(string?, IFormatProvider?)
ISpanFormattable (.NET 6+)	TryFormat(Span<char>, out int, ReadOnlySpan<char>, IFormatProvider?)
IParsable<TSelf> (.NET 7+)	Parse(string, IFormatProvider?), TryParse(string?, IFormatProvider?, out TSelf)
ISpanParsable<TSelf> (.NET 7+)	Parse(ReadOnlySpan<char>, IFormatProvider?), TryParse(ReadOnlySpan<char>, IFormatProvider?, out TSelf)
IUtf8SpanFormattable (.NET 10+)	TryFormat(Span<byte>, out int, ReadOnlySpan<char>, IFormatProvider?)
IUtf8SpanParsable<TSelf> (.NET 10+)	Parse(ReadOnlySpan<byte>, IFormatProvider?), TryParse(ReadOnlySpan<byte>, IFormatProvider?, out TSelf)

Comparison operators

Both structs now define <, <=, >, >= operators delegating to CompareTo.

Implicit conversion operators

Conversion	Direction
SequentialGuid ↔ Guid	Both directions
SequentialGuid ↔ string	Both directions
SequentialSqlGuid ↔ Guid	Both directions
SequentialSqlGuid ↔ string	Both directions

Updated README

Each package-level README now documents the struct wrappers. A new top-level README.md serves as the repository landing page with CI/NuGet badges and per-package summaries, while each project folder contains its own README packed into the NuGet package.

[Copilot]

Pull request overview

This PR promotes SequentialGuid and SequentialSqlGuid to richer, BCL-integrated value types by introducing a shared ISequentialGuid<TSelf> contract, adding parsing/formatting APIs across TFMs, adding comparison operators, and updating/readjusting README packaging and documentation across the repo.

Changes:

• Add ISequentialGuid<TSelf> with conditional formatting/parsing interface inheritance and (net7+) static abstract Create(Guid).
• Implement comparable/equatable/formattable/parsable members, comparison operators, and implicit conversions on SequentialGuid / SequentialSqlGuid.
• Add new per-package READMEs, update root README, and change packing to include project-local README.md in NuGet packages.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/SequentialGuid/ISequentialGuid.cs	Introduces shared interface contract with TFM-conditional interface inheritance and factory.
src/SequentialGuid/SequentialGuid.cs	Adds interface implementations, operators, implicit conversions, and parse/format APIs.
src/SequentialGuid/SequentialSqlGuid.cs	Adds interface implementations, operators, implicit conversions, and parse/format APIs for SQL-byte-order wrapper.
test/SequentialGuid.Tests/SequentialGuidStructTests.cs	Adds tests for Parse/TryParse and span formatting/parsing across TFMs.
test/SequentialGuid.Tests/SequentialSqlGuidStructTests.cs	Adds tests for Parse/TryParse and span formatting/parsing across TFMs.
src/SequentialGuid/README.md	Adds a full package README including new wrapper documentation.
src/SequentialGuid.EntityFrameworkCore/README.md	Adds per-package README for EF Core integration.
src/SequentialGuid.MongoDB/README.md	Adds per-package README for MongoDB integration.
src/SequentialGuid.NodaTime/README.md	Adds per-package README for NodaTime integration.
src/Directory.Build.props	Switches NuGet README packing to use project-local README.md.
README.md	Reworks root README into repo landing page with package index and summaries.

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