[NakaokaRei]

Problem

C++-enabled Swift consumers import different JNI types from Android's jni.h than a runtime compiled in C mode. Accessing imported function-table fields or constructing an object-valued jvalue in the consumer can also expose those C++ types, causing type mismatches.

This extracts the low-level helpers from swiftlang/swift-java#927 into JNI Core, following the review feedback.

Solution

• Add JNITypes aliases and an object-value factory defined in the runtime's C import context.
• Expose typed JNI function-table accessors through JNIEnvironment.swiftInterface so consumers preserve those signatures without casts.
• Add a C++-enabled consumer test target that calls Java String.concat, passing an object argument through jvalue and converting the returned reference.

Testing

• Core tests on macOS, including the C++ consumer test.
• Android ARM64 cross-build of the C++ consumer test target.
• Integration with the companion swift-java changes: 840 package tests; JavaKit normal/C++ modes (17 tests each); JNI sample normal/C++ modes (273 tests each); Android ARM64 JNI sample build with C++ enabled.

Integration checks used a local dependency override. The companion changes in swift-java will consume these helpers after they are available upstream.