(PATCH): Add an internal Reflection.Emit patch class builder

[PaulTrampert]

Closes #128

What

EmitPatchClassBuilder is a new internal builder. From the shared PatchClassModel, it emits the same patch class that PatchClassBuilder compiles with Roslyn, and it also works for internal source types whose assembly declares [InternalsVisibleTo("PTrampert.SimplePatch.Emitted")]. Nothing in the library calls it yet. PatchClassBuilder.Instance still uses Roslyn, and the opt-in is #95. The public API doesn't change, so this is a PATCH.

Points the diff doesn't make obvious

• The access grant. This follows Castle DynamicProxy. Each source type gets its own dynamic assembly (AssemblyBuilderAccess.Run), created inside AssemblyLoadContext.EnterContextualReflection(type.Assembly) so it lands in the source type's ALC. Every one of them is named PTrampert.SimplePatch.Emitted (EmitPatchClassBuilder.AssemblyName), so a consuming assembly grants access with [InternalsVisibleTo]. InternalsVisibleTo doesn't reach private or protected members, so private nested source types and private getters aren't supported.
• Accessibility check. Before emitting, EnsureAccessible walks every type the class names: the source type and its declaring types and generic arguments, plus each patched property's type and declaring type. It also checks every getter that Patch reads. If one isn't reachable, it throws NotSupportedException naming the assembly that needs the grant. Otherwise the runtime would fail at type load with an error that doesn't say how to fix it.
• Patch IL. Each value is _P.HasValue ? _P.Value : target.P, and target.P is read only on the false branch. A record is callvirt <Clone>$ + castclass, then dup/callvirt on each init accessor. A class is newobj with the bound values, then dup/callvirt on each setter. A struct is built in a local (newobj+stloc, or initobj), set with ldloca/call, and target is read through ldarga. Ignored properties are copied only for non-records, as in the Roslyn builder.
• Type name. It is {Namespace}.Optionals.{type.Name}_Optionals. Each type has its own assembly, so the name needs no random suffix and no identifier cleanup.
• A property with no getter throws NotSupportedException with a clear message. Roslyn gave a compile error for the same case.

Tests

• PatchClassBuilderTest is now a [TestFixtureSource] over both builders (Roslyn, Emit), so every existing regression case also covers the Emit path. Those cases now deserialize straight into the fixture's patch type. Going through IPatchObject<T> would hit PatchJsonConverterFactory, which always uses Instance.
• The cases that only apply to Roslyn moved unchanged to RoslynPatchClassBuilderTest: the cross-builder cache, concurrent first use, and the "must be public" exceptions.
• New shared case: target.P is not read when the patch sets P (ReadTrackingTestObject).
• New EmitPatchClassBuilderTest cases. The test project grants InternalsVisibleTo("PTrampert.SimplePatch.Emitted").
  • Supported:
    • an internal class (explicit null, [JsonPropertyName], init, an internal [JsonConverter])
    • [Range] validation of an internal class
    • an internal primary-constructor class with a get-only property
    • an internal struct
    • an internal type as a generic argument (List<InternalStruct>)
  • NotSupportedException:
    • a private nested class
    • a private positional record
    • a private getter
    • an internal property type from a second assembly that grants nothing, with the error naming that assembly
• That second assembly is a new test-helper project, PTrampert.SimplePatch.Test.External. It is unpacked and gives InternalsVisibleTo to the core test project, but not to the emitted assemblies.
• Mutation checks:
  • Removing the test project's InternalsVisibleTo fails the 8 tests on supported internal types.
  • Skipping EnsureAccessible fails the 4 NotSupportedException tests.

Alternatives rejected

• [IgnoresAccessChecksTo], which needs no setup from users and covers private nested types. It is undocumented and not officially supported (Document IgnoresAccessChecksToAttribute dotnet/runtime#37875), so review rejected it.
• Turning off Roslyn's accessibility check through compiler internals. That could break on any Microsoft.CodeAnalysis update.

Not covered

• Strong-named consumers. They can't name an unsigned friend assembly (CS1726). Castle handles this by giving its dynamic assembly a published public key. Tracked in Give the emitted patch assemblies a public key so strong-named assemblies can grant InternalsVisibleTo #131.

Results

• dotnet build: 0 errors. The warning set is unchanged: the same set of warnings as on main.
• dotnet test: all passing. Core 124/124 (was 86), Swashbuckle 17/17, OpenApi 11/11. This machine has no .NET 8 runtime, so the net8.0 test hosts ran with DOTNET_ROLL_FORWARD=Major on .NET 10.

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[github-actions]

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