Refactor into packages, modernize deps, and add tests

[JohanLindvall]

Restructure the codebase and bring it up to date:

• Split package main: business logic moves into an app package (no global state), the HTTP registry server into a registry package with a route table, and tag-file handling into a tagfile package. main.go is now just flag parsing and dispatch.
• Split the 500-line diz.go into archive/copy/manifest/registrymanifest files with clearer names; fix a bug in Merge that dropped all but the last new repo tag when coalescing manifests.
• Fix the Docker-Content-Digest response header (was double hex encoded).
• Update deflatewriter.go to the latest klauspost/pgzip, kept as a raw DEFLATE fork with every divergence documented.
• Drop io/ioutil; replace the local str package with stdlib slices.
• Update dependencies to latest and migrate to the modern docker client API, which lets the go.mod replace directive be removed.
• Add a Makefile and tests across diz, dockerref, hashzip, imagesource, app, tagfile, util and registry (coverage ~67%).