Initial `oxql` metrics explorer

Want to add a system-level page that let's the user explore metrics and construct oxql queries.

Acting at least initially as:

1. A way to learn and experiment with oxql
   1. Educational to understand query shapes
   2. Discover available schemas
2. An ad-hoc troubleshooting workflow
   1. Perhaps initially a smaller set of predefined query shapes; e.g. sled CPU, sled networking, instance CPU
   2. Less concerned about pure utility here to begin with, we can always follow up to optimise troubleshooting workflows later

UX

Probably a text-first or hybrid query surface. Might punt on a bi-directional UX unless there's a simple robust way to do it. Provide lightweight controls that write into the query rather than a fully separate visual builder.

Adding presets for queries and time-ranges.

Visualisation

Constrained set of chart presentations selected by metric schema/type/unit. Either using the data type to infer the chart type, or just getting Claude to make a map of the schemas → chart type. Maybe we'll need to clamp the range, or reduce the points. Manual refresh rather than polling. We could persist the query state in the URL param or add a share button that generates a link even if we're not adjusting the param of the active query.

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Will's lovely demo is a useful reference here: https://github.com/oxidecomputer/oxql-vis