Register New Zealand Microcosm releases in the calibration dashboard

[MaxGhenis]

Goal

Register New Zealand in the spec-driven Microcosm calibration dashboard so a valid policyengine/populace-nz release can render without country-specific UI branches.

Scope

• Add the NZ repository, labels, public visibility, geography, and deployment override variables to COUNTRY_REGISTRY.
• Expose calibration, targets, and comparison capabilities only.
• Add parser, selector, and capability regression coverage.
• Keep staging, model/runtime, pipeline, cross-dataset, and external-check pages fail-closed until producers publish those contracts.

Acceptance criteria

• ?country=nz resolves through the shared release-artifact reader.
• Selectors include New Zealand and existing countries remain unchanged.
• Missing or malformed NZ artifacts remain unavailable rather than synthesizing data.
• Frontend tests, typecheck, and build pass.