feat(stjm): source generator for reflection-free registration and generated discriminator members

[egil]

Follow-up to #207, #210 and #209. Do not start before #207 has landed and #210 has an explicit registration API; the generator emits into that API.

What a generator buys

Analysis result (see the linked issues for the measurements): a source generator does not improve the steady-state read path. Removing the discriminator probe and the second ReadStack would require generating a complete object deserializer (naming policies, case-insensitivity, ctor binding, required/init, extension data, number handling, callbacks, unmapped members), which STJ's own generator refuses to do for the same reason. That option is rejected.

What it does buy:

1. Discovery without reflection. Emit the AddMigratable<T>() / AddStaticMigration<,>() / AddExternalMigration<,,>() calls from docs(stjm): document trimming/NativeAOT limits, annotate reflection entry points, then add trim-safe explicit registration #210 for every [JsonMigratable] type and IMigrate<,> implementation in the compilation, replacing RegisterMigratorsFromAssembly. Cross-assembly types come through the referenced assemblies' own generated registrations (chainable), not by scanning references.

2. Discriminator as a real member. For partial [JsonMigratable] types, emit the discriminator property so the type's JsonTypeInfo is never modified and STJ's fast-path serialization applies to the migratable type itself (after perf(stjm): register migration through the resolver chain so source-gen fast-path serialization survives #207 only nested and unrelated types regain it):

   partial record UserV2
   {
       [JsonPropertyName("$type"), JsonPropertyOrder(int.MinValue)]
       public string JsonMigrationDiscriminator => "user-v2";
   }

   JsonMigratableConverterFactory.AddDiscriminatorProperty already early-returns when the property exists. Only attribute-declared values can be baked in; SetTypeDiscriminatorPropertyName and GetTypeDiscriminatorFrom are runtime configuration and must either be rejected when a generated member exists or take precedence with a diagnostic. Document that users can hand-write the same member today.

Shape

• Incremental generator (ForAttributeWithMetadataName), netstandard2.0, same project layout and Verify snapshot testing as Egil.StronglyTypedPrimitives; packed as an analyzer asset alongside the analyzer from feat(stjm): analyzer foundation and migration diagnostics #209 (same package id).
• Opt-in entry point mirroring STJ: [JsonMigrationContext] partial class AppMigrations producing AppMigrations.Default with a Configure(JsonMigrationBuilder) method; builder.AddContext(AppMigrations.Default).
• .NET 11 unions: emit nothing special; routing stays runtime (UnionCaseRouting reads cases from JsonTypeClassifierContext).
• Benchmarks: add a source-gen-registered variant of the existing scenarios and confirm the migratable root type serializes on the fast path with the generated member.