Add area quality gate

Goal

Add a lightweight check:area command that turns diagnostics and manifests into pass/fail quality gates.

Add a lightweight check:area npm command for an area quality gate.
The command must not run QGIS, Blender, Cesium, compression, or rebuild stages.
The command must reuse the same diagnostics source of truth used by diagnose:area; avoid duplicating OSM parsing, artifact checks, GLB budget checks, or manifest stale checks.
The first quality gate must fail on:
- invalid or missing OSM bounds
- malformed building multipolygon relations
- missing baseline GLB / metadata / Cesium preview artifacts
- invalid metadata JSON
- GLB size / node / image budgets exceeded
- missing, invalid, or stale expected stage manifests
Missing QGIS preview remains a warning in this first version.
The command must print a concise pass/fail report and use exit code 0 for pass and 1 for fail.
README and pipeline spec must document command usage and failure policy.

npm run check:area -- --config config/areas/nantaizi-lake-innovation-valley.json passes against current nantaizi outputs.
The check report includes pass/fail counts and any warning lines.
check:area exits non-zero for a deliberately impossible missing-output config or equivalent controlled failure case.
diagnose:area still works after any shared diagnostics refactor.
Syntax checks pass for changed Node scripts.
README and Trellis pipeline spec document check:area.

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.