Metadata bundle: lazy-load key features and data availability #1003

All pathway metadata is bundled and loaded up front on every page (import.meta.glob in src/data/pathwayMetadata.ts). With #1000 the metadata JSON grows from 0.90 MB to 4.76 MB. 3.09 MB of that is the 21 NGFS pathways, each carrying about 140 KB of key features (about 1,450 scoped entries per file).

Most pages need only the summary fields: name, publisher, type, sectors, regions and metrics. keyFeatures and dataAvailability are needed only on a pathway's detail page and by some search filters.

Options to evaluate:

• Split each pathway into a summary that loads eagerly and a detail chunk that is lazy-loaded with a dynamic import.
• Precompute the search facets the filters need, so search doesn't require the full key features.

Raised in review of #1000.