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/generic/remote.md

@@ -0,0 +1,28 @@
+# `generic` remote
+
+Shared classification, TTL and eviction rules: [caching](../common/caching.md).
+
+| Type | Mutable (auto-detected) | Immutable (auto-detected) |
+|---|---|---|
+| `generic` | nothing | everything |
+
+## Terraform
+
+```hcl
+resource "artifactapi_remote_generic" "github" {
+  name     = "github"
+  base_url = "https://github.com"
+
+  immutable_ttl = 0
+  mutable_ttl   = 7200
+
+  patterns = [
+    "ducaale/xh/.*/xh-.*-x86_64-unknown-linux-musl.tar.gz$",
+    "mikefarah/yq/.*/yq_linux_amd64$",
+  ]
+
+  mutable_patterns = [
+    ".*/archive/refs/heads/.*\\.tar\\.gz$",
+  ]
+}
+```