refactor: organize direct FFI backends

[DjDeveloperr]

Summary

• Reorganize NativeScript/ffi so Hermes owns the public JSI entrypoint, direct-engine bridge internals live under ffi/direct, and shared code is named for what it actually shares.
• Move direct adapters for V8, JSC, and QuickJS off facebook::jsi naming and onto nativescript::direct while keeping Hermes as the only real JSI backend.
• Centralize signature-dispatch hashing/lookup support, add direct prepared dispatch for V8/JSC/QuickJS, and wire Hermes/RN TurboModule staging to generated signature dispatch.
• Update CMake, build scripts, RN podspec packaging, and FFI boundary checks to enforce the new architecture.

Validation

• ./scripts/check_ffi_boundaries.sh
• git diff --check
• ./scripts/build_metadata_generator.sh
• BUILD_SIMULATOR=false BUILD_IPHONE=false BUILD_MACOS=true BUILD_VISION=false BUILD_CATALYST=false ./scripts/build_nativescript.sh --macos --no-iphone --no-simulator --jsc --ffi-direct --gsd-jsc
• BUILD_SIMULATOR=false BUILD_IPHONE=false BUILD_MACOS=true BUILD_VISION=false BUILD_CATALYST=false ./scripts/build_nativescript.sh --macos --no-iphone --no-simulator --quickjs --ffi-direct --gsd-quickjs
• BUILD_SIMULATOR=false BUILD_IPHONE=false BUILD_MACOS=true BUILD_VISION=false BUILD_CATALYST=false ./scripts/build_nativescript.sh --macos --no-iphone --no-simulator --v8 --ffi-direct --gsd-v8
• MACOS_TEST_ENGINE=hermes MACOS_TEST_FFI_BACKEND=direct MACOS_TEST_GSD_BACKEND=hermes ... node scripts/run-tests-macos.js build/test-results/macos-hermes-gsd-on-junit.xml -> 713 specs, 0 failures, 8 skipped
• MACOS_TEST_ENGINE=hermes MACOS_TEST_FFI_BACKEND=direct MACOS_TEST_GSD_BACKEND=none ... node scripts/run-tests-macos.js build/test-results/macos-hermes-gsd-off-junit.xml -> 713 specs, 0 failures, 8 skipped
• ./scripts/build_react_native_turbomodule.sh --no-pack

The metadata generation steps still print existing SDK/private-header diagnostics, but the commands exit successfully.

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