Add area diagnostics command

Goal

Add a fast area diagnostics CLI for OSM structure and existing output artifact health.

Add a fast diagnostics CLI for an area config.
The command must not invoke QGIS, Blender, Cesium, or GLB compression.
The report must scan the configured OSM XML for common manual-edit risks: bounds presence, nodes, ways, relations, building ways, building multipolygon relations, explicit heights, building levels, malformed relation members, unresolved member ways, open rings, and basic vegetation counts.
The report must inspect existing output artifacts when present: GeoJSON directory, GeoPackage, QGIS project, .blend, render PNG, GLB, metadata, Cesium preview, compressed GLB, compressed metadata, and compressed preview.
If a GLB exists, the report should reuse the existing GLB digest logic to summarize size, nodes, meshes, materials, images, accessors, and extensions.
The command should emit clear warnings for likely trouble, including missing OSM bounds, malformed building multipolygons, missing expected artifacts, GLB over a conservative size budget, high node count, and high texture count.
Add an npm script and README usage.

npm run diagnose:area -- --config config/areas/nantaizi-lake-innovation-valley.json prints a useful text report without running heavy build stages.
The diagnostics report includes OSM structure, building relation health, vegetation counts, output artifact status, and GLB digest summary.
Missing optional output artifacts are warnings, not hard failures.
Syntax checks pass for the changed Node scripts.
README documents the new diagnostic command.

Keep prd.md focused on requirements, constraints, and acceptance criteria.
Lightweight tasks can remain PRD-only.
For complex tasks, add design.md for technical design and implement.md for execution planning before task.py start.