Add Castle.customize/1 as the public release integration API

[ausimian]

Summary

Expose a single public API for making a normal Mix release Castle-capable, rather than requiring users to know about Forecastle release steps directly.

Motivation

Today Forecastle integration requires consumers to explicitly place Forecastle.pre_assemble/1 and Forecastle.post_assemble/1 around :assemble. That leaks the implementation package and makes future release integration changes part of the consumer API.

The desired user-facing shape is approximately:

defp releases do
  [
    my_app: fn ->
      [
        steps: [:assemble, :tar]
      ]
      |> Castle.customize()
    end
  ]
end

The lazy release function is useful because it ensures Castle/Forecastle dependencies have been compiled before the customization code is invoked.

Proposed behaviour

Castle.customize/1 should accept ordinary Mix release options and return ordinary Mix release options with Castle integration applied. Internally it may delegate to Forecastle.

At minimum it should:

• install the Forecastle pre/post assembly steps around :assemble;
• preserve any existing custom steps and their ordering;
• remain the sole public integration point for consumers;
• leave room for future Castle-specific release customization without changing user mix.exs files.

Forecastle should remain an implementation detail for most users.

Acceptance criteria

• A consumer can opt into Castle release support with a single Castle.customize/1 call.
• Existing custom release steps are preserved.
• Forecastle-specific functions no longer need to appear in documented consumer release definitions.
• Documentation shows the lazy release-definition form to avoid dependency-loading problems while evaluating mix.exs.