Add multiarch build setup and build documentation

- Add `make setup` target to register qemu and create a multiplatform
  buildx builder for universal (x86_64 + aarch64) packages
- Use named builder with --progress=plain to avoid flickering output
- Conditionally depend on arch-specific Docker tars so single-arch
  builds don't require the other image
- Document prerequisites, build targets, and overrides in README

README.md

@@ -2,25 +2,57 @@
 
 StartOS 0.3.5.1 wrapper for [Kamado Pool](https://github.com/Relaxo143/KamadoPool), a modern solo Bitcoin mining pool built on a patched fork of CKPool-solo with a Go middleware API and Svelte real-time dashboard.
 
-## Build
+## Prerequisites
+
+- **Docker** with [buildx](https://docs.docker.com/build/buildx/) plugin
+- **deno** — bundles the TypeScript embassy procedures
+- **yq** — parses `manifest.yaml` for package metadata
+- **start-sdk** — packs and verifies the `.s9pk` (from the [StartOS SDK](https://docs.start9.com/latest/developer-docs/specification/))
+- **KamadoPool source** — local sibling checkout at `../KamadoPool` (override with `KAMADO_SRC=/path/to/KamadoPool`)
+
+## Building
+
+### Universal package (x86_64 + aarch64)
+
+A universal `.s9pk` runs on any StartOS machine regardless of architecture. This is the default and what you should ship.
+
+**One-time setup** (re-run after each reboot):
+
+```sh
+make setup
+```
+
+This registers qemu binfmt handlers for cross-architecture emulation and creates a multiplatform Docker buildx builder. The arm64 build runs under emulation on x86 hosts, so it is significantly slower (~5-10x) than native.
+
+**Build:**
 
 ```sh
 make
 ```
 
-This runs `deno` to bundle the embassy TypeScript procedures, builds a multi-arch OCI image via `docker buildx`, and packs everything into `kamado-pool.s9pk` using `start-sdk`.
+This bundles the TypeScript procedures, builds Docker images for both architectures, and packs `kamado-pool.s9pk`. The universal package is roughly double the size of a single-arch package since it contains two Docker images.
 
-The build pulls KamadoPool source from a **local sibling checkout** (`../KamadoPool` by default) via a docker buildx named build context — no GitHub clone, no pinned SHA. If your checkout lives elsewhere, override it:
+### Single-arch (development)
+
+For faster iteration when you only need one architecture:
 
 ```sh
+make x86    # x86_64 only
+make arm    # aarch64 only
+```
+
+These skip the other architecture entirely — no emulation overhead.
+
+### Overrides
+
+```sh
+# Use a different KamadoPool source directory
 make KAMADO_SRC=/path/to/KamadoPool
+
+# Use a different buildx builder
+make BUILDER=my-builder
 ```
 
-## Install
-
-```sh
-make install
-```
 
 ## License