metal3: Add BareMetalHost plugin with list and detail views

metal3/.gitignore

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+node_modules
+.vscode
+dist
+.eslintcache

metal3/README.md

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+# Metal3 Headlamp Plugin
+
+A [Headlamp](https://headlamp.dev) plugin that adds a Metal3 section to the UI for viewing
+bare-metal infrastructure through Kubernetes.
+
+[Metal3](https://metal3.io) manages bare-metal hosts as Kubernetes resources. This plugin surfaces
+those resources in Headlamp with list and detail views, so operators can inspect their bare-metal
+estate without dropping to `kubectl`.
+
+> Status: early / in progress. See [Current scope](#current-scope) and [Limitations](#limitations).
+
+## Current scope
+
+- **BareMetalHost**
+  - List view with a composite **Status** column that combines the host's operational status
+    (the headline signal), its provisioning state, and any error type into a single cell. A host
+    that is provisioned but in error is therefore shown as both at once, which a single-value
+    column cannot do.
+  - Detail view with configuration, status, consumer, power state, and BMC fields.
+- Graceful handling when the Metal3 CRDs are not installed in the connected cluster: the views
+  show an explanatory message instead of an empty or erroring screen.
+
+## Prerequisites
+
+- [Headlamp](https://headlamp.dev) (desktop app or in-cluster).
+- A Kubernetes cluster whose API has the Metal3 `baremetal-operator` CRDs installed, in
+  particular `baremetalhosts.metal3.io`. If those CRDs are absent, the plugin indicates that the
+  operator is not installed rather than showing an empty view. To stand up Metal3 and provision a
+  host, see the Metal3 [Try it](https://book.metal3.io/developer_environment/tryit) getting-started
+  guide.
+
+## Limitations
+
+- Read-only for now: no mutating actions (power on/off, reboot, detach) yet.
+- Only the BareMetalHost resource is implemented. The cluster-api-provider-metal3 resources
+  (Metal3Machine, Metal3Cluster, and the rest), the resource-relationship map, and mutating
+  actions are planned.
+
+## Development
+
+Install dependencies and start a watch build:
+
+```bash
+npm install
+npm start
+```
+
+Then run Headlamp (the desktop app or a build from source) pointed at a cluster that has the
+Metal3 CRDs; the plugin's "Metal3" entry appears in the sidebar.
+
+A sample BareMetalHost manifest for local testing is in
+[`test-files/baremetalhost.yaml`](./test-files/baremetalhost.yaml):
+
+```bash
+kubectl apply -f test-files/baremetalhost.yaml
+```
+
+Other useful commands:
+
+```bash
+npm run build   # production build to dist/
+npm run tsc     # type-check
+npm test        # unit tests (vitest)
+npm run lint    # lint
+```
+
+For general Headlamp plugin development, see the
+[plugin documentation](https://headlamp.dev/docs/latest/development/plugins/).
+
+## License
+
+Apache License 2.0.