Export every public drag and download host ABI function #633

Follow-up to #625/#626 and #631/#632.

Problem

Authoritative current-main architecture run 35430888338 fails NativeAbiExportTests.MacOsExportListMatchesEveryPublicAbiFunction. Six public typed drag/download ABI declarations are absent from webscene_native_engine.exports:

• webscene_engine_dispatch_drag_v1
• webscene_engine_take_download_request_v1
• webscene_download_request_release_v1
• webscene_engine_take_outbound_drag_request_v1
• webscene_outbound_drag_request_release_v1
• webscene_engine_complete_outbound_drag_v1

The #631 assumption that four were deliberately header-only was incorrect. Every non-instrumentation WEBSCENE_API declaration is required in the macOS export manifest.

Fix

Export all six public functions. Classify all six as independently versioned exported host services in the tooling contract, remove the header-only exception, and correct the validation record.

Acceptance

• Public ABI declarations and export manifest are exact sets.
• All six drag/download functions remain required in header and exports.
• Unknown v1/v2 JavaScript interop symbols remain rejected.
• Linux and macOS architecture/tooling gates pass.