feat: improve api for migration registration

[subomi]

Summary

This PR refines migration registration into an explicit build lifecycle and tightens runtime behavior.

• Introduces a fluent registration API: rm.Register(Migration[T](...)).Build().
• Moves eager graph creation to Build() (instead of registration-time side effects).
• Streams typed <-> generic JSON conversion via io.Pipe helpers to avoid extra full-buffer allocations.

Breaking API changes

• Replaces the standalone Register[T](rm, version, migration) error flow with:
  • Migration[T](version, migration)
  • (*RequestMigration).Register(...VersionedTypeMigration) *RequestMigration
  • (*RequestMigration).Build() error
• For/Bind now require a prior Build() call and return ErrNotBuilt if called early.
• Registering more migrations after build now returns ErrAlreadyBuilt.

Behavior updates

• Build() now:
  • validates accumulated registration state,
  • sorts versions by configured format,
  • eagerly builds/caches type graphs for registered types,
  • marks the migrator immutable for request-time usage.
• Runtime lazy graph fallback remains for unregistered container/interface-driven shapes.

Test plan

• Updated existing tests to the new Register/Build lifecycle.
• Added/updated coverage for build-gating (ErrNotBuilt) and eager cache-at-build expectations.
• go test ./...