Castle.customize/1: an upgrade_from: option

[ausimian]

The release-facing half of folding relup generation into assembly (ausimian/forecastle, "Generate the relup during assembly").

Castle.customize/1 already owns step arrangement — it puts Forecastle's steps around :assemble and defaults missing steps to [:assemble, :tar]. It grows an option naming one or more baselines:

my_app: fn ->
  [include_executables_for: [:unix]]
  |> Castle.customize(upgrade_from: ["tar:artifacts/my_app-1.0.0.tar.gz"])
end

When set, the relup-generating step is inserted between post_assemble and :tar, and the double build disappears.

Notes

• Baseline specs are the shared grammar (rel:, tar:, ref:), resolved by Forecastle.
• Dynamic values are already available without new machinery: the release definition is a fn -> thunk, so mix.exs can read the list from the environment or compute it.
• One accepted tension. With ref:, an ordinary mix release triggers a minutes-long build of a previous version as a side effect. It is opt-in and tar: is fast, so it stands — but it is what would later justify a mix castle.baseline <spec> task so CI can run the resolution as its own cacheable stage.

Acceptance

• upgrade_from: with one and with several baselines.
• Omitting it leaves assembly exactly as it is today.
• Documented in the README's "Appups and relups" section, which currently prescribes the double build without saying so.

Rationale: design/upgrade-tooling.md §D5.