Refine external class resolution and deferred methods

[t-kalinowski]

Summary

• Tighten external class resolution so new_external_class() matches an S7
  class bound to the requested name and package, with clearer failure messages.
• Improve external-class behavior in property inheritance, union matching,
  version checks, and validation errors.
• Simplify deferred external method registration/unregistration and hook cleanup
  for external classes and generics.
• Update tests, snapshots, and the package vignette to reflect the narrower
  external-class contract.

Public-facing changes

• new_external_class() now documents and reports the class binding contract
  more directly.
• External-class methods and unions behave more consistently when dependency
  packages load, unload, or are not yet available.

Internal-only changes

• Refactors dependency collection, hook registration/removal, external generic
  sentinels, and deferred method cleanup without adding exported API.