Split per-type registration points into per-type files (#150)

Parallel package-type PRs conflict because every type edits the same shared lines (README tables, provider.go, server.go imports, the package type list, UI maps, the e2e caching table). A new type now only adds files.

- move per-type README sections into docs/<type>/{remote,local,virtual}.md
- split provider.go capability and metadata types into cap_*.go and metadata_*.go
- import providers via internal/provider/all, one file per type
- declare each package type in package_type_<type>.go, registered via init()
- split the e2e caching cases and UI colours/snippets into per-type files

Reviewed-on: #150
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

docs/docker/local.md

@@ -0,0 +1,17 @@
+# Local docker registry
+
+A local `docker` repo is a real container registry, not a mirror: it serves the
+Docker Registry HTTP API V2 for both push and pull, so any client (`docker`,
+`podman`, `skopeo`, `buildah`) can use it directly.
+
+```sh
+docker tag myapp:latest artifactapi.example.com/docker-internal/myapp:latest
+docker push        artifactapi.example.com/docker-internal/myapp:latest
+docker pull        artifactapi.example.com/docker-internal/myapp:latest
+```
+
+The first path segment after `/v2/` is the artifactapi repo name; the remainder
+is the image name. Blobs and manifests are stored through the shared
+content-addressable store (deduplicated by digest, reaped by GC once
+unreferenced); tags are mutable references and re-pushing a tag moves it. Blob
+uploads support both the monolithic and chunked (`POST`/`PATCH`/`PUT`) flows.