README documents an appup compiler and `mix castle.relup` task that no longer live in Castle

[ausimian]

Problem

The README describes build-time tooling as part of Castle, but that support was moved out to Forecastle in 0.3.0 (see CHANGELOG: "Move build time support into Forecastle"). The repo now contains only lib/castle.ex — there is no Mix compiler and no Mix task (grep for Mix.Task/defmodule Mix.Tasks returns nothing).

Stale sections:

• "The Appup Compiler" (README ~L81-116) documents adding [:appup] to compilers:, implying the compiler ships in Castle.
• "Relup Generation" (README ~L118-129): "Castle contains a mix task, castle.relup…" — it does not; mix castle.relup is not defined in this project.
• Installation (README ~L15-16): "For projects that don't define a release, but use the appup compiler, it's sufficient to bring Castle in as a build-time dependency" — implies the compiler is provided by Castle.

Impact

Users following the README will try to use :appup / mix castle.relup from Castle and find they don't exist, with no pointer to where they actually live.

Suggested fix

Either move these sections to the Forecastle docs and link out from Castle's README, or clearly state that the appup compiler and castle.relup task are provided by Forecastle. Verify which package actually owns each piece and update accordingly.

[ausimian]

The task rename decided in #32 changes how this resolves.

Rather than correcting the README to say mix forecastle.relup, the tasks are being renamed back to the castle.* namespace — implemented in Forecastle, but named for the package people actually depend on (ausimian/forecastle#24). The packaging split is a build-time concern; the task namespace is a user-interface one, and making them agree is what produced forecastle.relup in the first place.

So the README becomes correct roughly as originally written, plus a sentence saying the tasks are provided by Forecastle. The appup compiler stays mix compile.appup throughout — it is named by its :compilers entry, not by a package.

Reasoning in design/upgrade-tooling.md §D1 (lands with the first PR in this tree).