Improve template authoring with init, inspect, validate, and richer preview

[mintyPT]

Problem

Template authors need faster feedback than rendering manually and debugging layer names by hand. The current preview command emits a PNG, but the authoring loop should be more discoverable.

Proposal

Add template project scaffolding, template inspection, config validation, and a more useful local preview workflow.

Acceptance criteria

• Add clickclick init to scaffold a minimal template project.
• Add clickclick inspect <template> to list data-layer names and useful metadata.
• Add clickclick validate <config-or-template> with actionable diagnostics.
• Improve preview/watch so it is convenient during authoring.
• Include validation for missing layers, duplicate layers, invalid recipes, and missing assets.
• Add tests for init output, inspect output, and validation failures.
• Document the authoring workflow.