(MINOR): Add PatchClassBuilder.UseExperimentalDynamicClassBuilder

[PaulTrampert]

Why

IPatchObject<T> throws NotSupportedException for an internal T, because the Roslyn builder compiles the patch class into a separate assembly that can only name public types (#94). #128 added an internal Reflection.Emit builder that doesn't have this limit, and #133 put both builders behind IPatchClassBuilder, with PatchClassBuilder as a facade. This PR lets applications opt in to the Emit builder.

What changed

• New public static bool PatchClassBuilder.UseExperimentalDynamicClassBuilder { get; set; }, off by default. PatchClassBuilder.GetPatchClassFor reads it on every call and delegates to EmitPatchClassBuilder.Instance when it's on, or RoslynPatchClassBuilder.Instance when it's off. Instance keeps its type, so this is binary compatible.
• The flag is process-wide, so PatchJsonConverterFactory, the Swashbuckle filter and the OpenAPI transformer all follow it with no extra wiring. Its XML docs say to set it at startup, before first use: each builder keeps its own cache, but System.Text.Json and the OpenAPI generators cache the types they've already resolved.
• Both builder classes stay internal. The flag is the only new public API, so this is MINOR.
• The Roslyn builder's NotSupportedException for non-public types now tells you to set the flag.
• Docs: the flag's XML docs, the GetPatchClassFor <exception> docs, a new "Non-public write models (experimental)" section in README.md, a short section in docs/getting-started.md, and AGENTS.md ("How it works").

Alternatives rejected

• An AddSimplePatchConverters(options, bool) overload (Add an opt-in flag to use the Reflection.Emit patch class builder #95's original design). It hides a process-wide setting behind an API that looks per-options. It also needed a "can only be turned on" rule so that two option sets couldn't disagree.
• Retyping Instance to IPatchClassBuilder. This breaks binary compatibility, so it's deferred to Retype PatchClassBuilder.Instance to IPatchClassBuilder #135.
• A one-way Enable…() method, or a flag that locks after first use. Both are harder to test and add API surface. Documenting "set at startup" is enough for an experimental opt-in.

Tests

• Core (UseExperimentalDynamicClassBuilderTest, [NonParallelizable], resets the flag in TearDown):
  • The flag defaults to off.
  • With the flag on, Instance returns the Emit builder's type.
  • After the flag is turned back off, it returns the Roslyn builder's type again (separate caches).
  • With the flag off, an internal type throws an error that names the flag.
  • With the flag on, an internal type works end to end through AddSimplePatchConverters: patching, explicit null, an omitted property, and [Range] validation.
• Swashbuckle and OpenAPI: new tests generate a patch schema for an internal model with the flag on. Each test project grants InternalsVisibleTo("PTrampert.SimplePatch.Emitted"). I checked that both tests fail with NotSupportedException when the flag isn't set.
• dotnet build: no new warnings, and none come from changed files. dotnet docfx: only the existing warnings.
• dotnet test: core 129/129, Swashbuckle 18/18, OpenApi 12/12. The net8.0 projects were run with DOTNET_ROLL_FORWARD=Major because only the .NET 10 runtime is installed locally.

Closes #95

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