clarify WFS module boundaries and naming

[esgn]

closes #58

This PR clarifies WFS module boundaries and naming without changing behavior.

Included

• rename src/helpers/wfs_internal to src/helpers/wfs_engine
• rename src/gpf/wfs.ts to src/gpf/wfs_schema_catalog.ts
• update imports accordingly
• add explicit responsibility comments to distinguish:
  • the legacy/simple flattened WFS helper layer
  • the structured WFS engine used by MCP WFS tools

Scope

• no intended runtime behavior changes
• no broad WFS refactor
• no contract changes for existing tools

Validation

• npm run typecheck
• npm test -- --runInBand