feat(registry): add Nexus-compatible named Maven and npm registries

[ivaseeq]

What changed

• adds named Maven hosted, proxy, and group repositories with Nexus-compatible routes and policies
• adds separate npm hosted, proxy-cache, and ordered group repositories
• keeps npm hosted manifests, tarballs, tags, deprecations, package fields, and rebuildable packument cache separate from proxy state
• materializes hosted npm packuments so warm metadata reads use one object-store GET instead of one GET per package version
• invalidates the derived packument cache under the package mutation lock for publish, dist-tag, deprecation, and retention changes
• adds conditional immutable writes, retention, GC, indexing, metrics, UI, auth, and OpenAPI support for the named layouts

Why

This enables a fresh Nora installation to replace the developer-facing Maven and npm flows currently served by Nexus. Clients retain their standard Maven and npm commands; deployments change only registry endpoints and authentication material.

The packument cache fixes a migration and developer-flow failure observed with packages containing thousands of versions: every metadata request previously fanned out into thousands of S3 reads and exceeded the ingress timeout. The cache is derived and rebuildable; authoritative hosted state remains split by responsibility.

The target topology is a singleton writer. Migration from older Nora object layouts and active-active mutable metadata coordination are intentionally outside this change.

Validation

• cargo fmt --all -- --check
• cargo clippy -p nora-registry --all-targets -- -D warnings
• library tests: 63 passed
• binary tests: 1,797 passed, 1 ignored
• documentation tests: 6 passed, 1 ignored
• git diff --check
• targeted tests cover cold/warm packument reads, per-route tarball URL rewriting, mutation invalidation, fail-closed invalidation, retention, and proxy-cache isolation
• combined Maven/npm acceptance and live Nexus-to-Nora migration are tracked separately against the digest built from this source