perf(stjm): typed migrator invokers to remove boxing, per-call GetService and the discriminator scan #208

The migration path (not the happy path) goes through untyped plumbing that boxes and re-resolves on every call:

• IMigratorInvoker.TryMigrate(object? source, out object? migrated) boxes struct sources and struct targets, and does a source is TSource type test per call (StaticMigratorInvoker, ExternalMigratorInvoker).
• JsonMigratableConverter<T>.ReadSource reads the source through StjInternals.ReadAsObject, which boxes struct sources; the typed JsonConverter<TSource>.Read is available once the source type is known statically.
• ExternalMigratorInvoker.ResolveMigrator calls serviceProvider.GetService(migratorType) on every migration.
• FindMigratorByDiscriminator is a linear ValueTextEquals scan over context.Migrators.
• SetMigrationTracking does an is IJsonMigrationTracked interface test per read, including the happy path; a static readonly bool computed from typeof(T) folds away at JIT time.

Change

• Make the per-source reference generic: MigratorReference<TSource, TTarget> (created once at converter build time) exposing a typed bool TryRead(ref Utf8JsonReader, out TTarget) that calls JsonConverter<TSource>.Read and then TTarget.TryMigrateFrom (static abstract) or the external IMigrate<TSource, TTarget> instance directly. Keep a non-generic base for the discriminator/shape metadata the read path iterates over.
• Keep DI semantics (scoped migrators are resolved per call by design, see docs/recipes/dependency-injection.md) but avoid the GetService round trip when no service provider is configured, and avoid the Lazy<T> indirection on the fallback path.
• Replace the linear discriminator scan with a length-bucketed or first-byte-switched lookup when there are more than a handful of migrators; keep the single-migrator path branch-free.
• Cache the IJsonMigrationTracked check per T.
• Benchmarks: add a struct source/target scenario and a "many versions" scenario (for example 8 registered sources) so the boxing and scan changes are visible; refresh docs/perf/*.md.
• Coverage stays at 100% branch for core components per Egil.SystemTextJson.Migration/AGENTS.md.

This work is independent of #207 but touches JsonMigratableConverterFactory.BuildMigratorMap, so land it after or rebase on it.