feat: add experimental Node-API feature infrastructure

[kraenhansen]

Summary

Adds infrastructure to support experimental Node-API features in the CTS, enabling future test ports for the six test suites that depend on experimental APIs (test_dataview, test_sharedarraybuffer, test_object, test_general, test_finalizer, 6_object_wrap). Closes #26.

• Vendor Node-API headers from the Node.js repository (replacing the node-api-headers npm package), giving access to experimental API declarations behind #ifdef NAPI_EXPERIMENTAL
• Add scripts/update-headers.mjs to download latest headers and generate layered Windows .def files via clang AST dump
• Add add_node_api_cts_experimental_addon() CMake function that compiles addons with NAPI_EXPERIMENTAL defined
• Add implementor feature declaration (features.js harness module) so runtimes declare which experimental features they support via globalThis.experimentalFeatures
• Add harness validation test (tests/harness/features.js) that ensures all expected feature flags are declared as booleans

Design decisions

1. Vendored headers — Uses actual Node.js headers verbatim. NAPI_EXPERIMENTAL works exactly as upstream. Removable if node-api-headers ever adds experimental support.
2. NAPI_EXPERIMENTAL at C level, per-feature guards at JS level — The vendored headers auto-define all NODE_API_EXPERIMENTAL_HAS_* macros when NAPI_EXPERIMENTAL is set. Per-feature granularity is only needed in JS (experimentalFeatures.*).
3. Silent skip — When a runtime doesn't support a feature, guarded code simply doesn't execute; the test passes with zero experimental assertions.
4. Layered .def files — Four deduplicated .def files (js_native_api.def, node_api.def, js_native_api_experimental.def, node_api_experimental.def) combined at CMake configure time for Windows import libraries.

Test plan

• npm run addons:configure && npm run addons:build — addons compile with vendored headers
• npm run node:test — all 26 tests pass, including new harness/features.js
• npm run update-headers — headers download and .def files regenerate via clang
• Windows CI — MSVC builds with layered .def import libraries

🤖 Generated with Claude Code

Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com

[KevinEady]

Should the GitHub Actions workflow be extended to include experimental builds/tests?

[kraenhansen]

Should the GitHub Actions workflow be extended to include experimental builds/tests?

Not really as I see it. Running / skipping tests based on the availability of experimental features is controlled by the harness. I expect a future PR to update the value of the feature flags based on the Node.js version executing the harness.