Export: data source kind + fast schema (field names/types)

[danmaps]

Why

Proprietary packs need to avoid dumb recommendations and detect brittle automation.

Proposal

Data source kind

Add a normalized field per layer/table like:

• dataSourceKind: file_gdb | enterprise_gdb | service | shapefile | raster | unknown

Fast schema

Add optional "fast schema" field info (names/types only):

• fields: [{ name, fieldType, length?, isNullable?, domainName? }]

Acceptance Criteria

• Kind is present for layers/tables where dataSource is available.
• Field schemas can be exported in a fast mode (no sampling, no counts).
• Output is persisted to flat files under .arcgispro/context/.

Notes

This should NOT require GP history, map screenshots, or UI state.