Export profile clear API from the public C ABI #836

Problem

The V8-enabled native package build fails while linking webscene_native_engine_tests because webscene_profile_clear_data_v1 is declared as a public C ABI function but its definition is nested inside the scene-acquisition anonymous namespace.

Observed in the exact package gate for PR #835 (run 35509963213, Linux job 106076080246):

undefined symbol: webscene_profile_clear_data_v1
referenced by test_durable_profile_restart_contract

The shared-library translation unit compiles, but the function has internal linkage, so consumers cannot resolve the advertised ABI.

Proposed fix

• close the scene helper anonymous namespace before defining webscene_profile_clear_data_v1;
• keep the public function in the surrounding extern "C" block;
• add a regression that verifies the compiled object exposes the public symbol and does not retain a local anonymous-namespace copy;
• run the focused native package/link gate on Linux and Windows.

Acceptance

• webscene_native_engine_tests links with V8 enabled;
• durable profile restart/clear coverage passes;
• packaged native library exports webscene_profile_clear_data_v1 on supported platforms;
• no change to the ABI signature or profile clearing behavior.