Add Flow<DataClass> support to FFM bridge generator

[kdroidFilter]

Summary

• Plugin: Support DATA_CLASS as Flow element type in both NativeBridgeGenerator and FfmProxyGenerator
• Native side: Generates per-DC reader bridges (flowdc_{Name}_read) using StableRef + out-params pattern
• JVM side: Decodes DC fields from handle via generated FLOWDC_*_READ_HANDLE method handles
• GraalVM: Automatically registers downcall descriptors for reader bridges in reachability metadata
• Examples: Added memoryFlow() to systeminfo, and pointFlow/namedValueFlow/taggedPointFlow/calcResultFlow/rectFlow to calculator
• Tests: 30+ new tests covering simple DC, nested DC, DC with String fields, DC with enum fields, empty/single/large flows, error handling, concurrency, and cross-feature interactions

Test plan

• ./gradlew :examples:calculator:jvmTest — all flow DC tests pass
• ./gradlew :examples:systeminfo:compileKotlinLinuxX64 :examples:systeminfo:compileKotlinJvm — systeminfo builds
• Verify generated bridges contain flowdc_*_read functions
• Verify GraalVM reachability-metadata.json includes reader bridge downcall descriptors