Split public header packaging between framework and file set

[kraenhansen]

Context

CMake 4.2 (the current default on GitHub's macos-latest runners) rejects the weak-node-api build with:

CMake Error in CMakeLists.txt:
  The file set "HEADERS", of type "HEADERS", is incompatible with the
  "FRAMEWORK" target "weak-node-api".

packages/weak-node-api/CMakeLists.txt attached the public headers to the target two ways at once — a modern FILE_SET HEADERS and the framework's legacy PUBLIC_HEADER (inside the if(APPLE) block that also sets FRAMEWORK TRUE). CMake 4.2 makes a HEADERS file set incompatible with a FRAMEWORK target, so the Apple build now errors.

As a temporary unblock, #374 / #375 / #377 pinned CMake to 4.1.2 on five macOS jobs. This fixes the root cause and removes those pins.

Changes

packages/weak-node-api/CMakeLists.txt

• Hoisted the header list into a PUBLIC_HEADER_FILES variable (previously read back from the target's HEADER_SET property).
• On Apple, ship the public headers via the framework's PUBLIC_HEADER property only — no FILE_SET HEADERS on the framework target. The header list is identical to before, so XCFramework packaging is unchanged.
• On other platforms, keep the HEADERS file set for install packaging.
• Added an explicit target_include_directories(... $<BUILD_INTERFACE:...>) so in-tree consumers (the C++ tests, which #include <weak_node_api.hpp>) still get the include paths the file set used to supply implicitly.

.github/workflows/check.yml

• Reverted the macOS CMake 4.1.2 pin from all five jobs: unit-tests, weak-node-api-tests, test-ios, test-macos, test-ferric-apple-triplets.
• Kept the CMAKE_VERSION env var — it still selects the Android SDK cmake package in test-android, unrelated to this issue.

Verification

Validated the non-Apple path end-to-end locally: generated the sources, then cmake -S . -B build-tests -DBUILD_TESTS=ON → build → ctest, all passing — including the test's #include <weak_node_api.hpp>, confirming the new target_include_directories propagates includes to consumers.

The Apple .framework / XCFramework build is exercised by this PR's macOS CI (hence the weak-node-api / Apple 🍎 / Ferric 🦀 labels). All four Apple framework-building jobs — Unit tests (macos-latest), Weak Node-API tests (macos-latest), Test app (iOS), and Test ferric Apple triplets — pass on the runner's default CMake, confirming the pin is no longer needed.

Note: test-macos (the MacOS 💻 label) is intentionally not applied. It currently fails to build the React Native macOS test app due to an unrelated fmt consteval error in RN's own C++ (Yoga / logger), which has nothing to do with this change. That's tracked separately.

Acceptance criteria

• weak-node-api configures and builds on macOS with the runner's default CMake (no version pin) — validated by CI.
• Reverted the macOS CMake pin from all affected jobs.
• macOS .framework / XCFramework still ships the correct public headers (identical PUBLIC_HEADER list).

Closes #376

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