fix: String retry + suspend ByteArray support + unsupported features docs

[kdroidFilter]

Summary

1. String buffer retry (JetBrains-style)

• Replace hardcoded reinterpret(8192) with dynamic retry-on-truncation pattern
• Allocate initial 8KB, call native, if returned size >= buffer size, reallocate with exact size and retry
• Applies to all string/bytearray return paths: functions, properties, suspend, StableRef reads
• Extract appendStringReadWithRetry helper to DRY logic across codegen sites

2. Suspend ByteArray support (NEW)

• Add native readByteArrayRef helper (mirrors STRING pattern)
• Add BYTE_ARRAY case to suspend result encode/decode (NativeBridgeGenerator + FfmProxyGenerator)
• Add readByteArrayFromRef method + MethodHandle on JVM side (with retry)
• Enables: suspend fun(): ByteArray, suspend fun(): ByteArray?, large arrays >8KB

3. Comprehensive "What's NOT supported" documentation (NEW)

• Language features (interfaces, inheritance, generics, etc.)
• Suspend limitations (DataClass return, collections return)
• Flow limitations (ByteArray elements, collections elements)
• Callback limitations (nullable params, ByteArray param, lambda return)
• Collection/property/data class field limitations
• Each gap explains the reason + suggests alternative approach

Testing

• All existing tests pass (calculator + systeminfo)
• Added 7 new suspend ByteArray tests:
  • Basic return + state mutation
  • Nullable (null + non-null case)
  • Large arrays (16KB, exceeds initial 8KB buffer)
  • Concurrent calls (20 concurrent async tasks)

Files Changed

• NativeBridgeGenerator.kt — add readByteArrayRef helper + BYTE_ARRAY suspend encode
• FfmProxyGenerator.kt — add string retry helper, update all string return paths, add BYTE_ARRAY suspend decode + MethodHandle
• Calculator.kt — add 3 suspend ByteArray functions
• SuspendFunctionTest.kt — add 7 suspend ByteArray tests
• README.md — add detailed "What's NOT supported" section with 3 tables

Test Plan

• Plugin compiles
• calculator:jvmTest (780+ tests pass, including 60 suspend)
• systeminfo:jvmTest passes
• Strings >8KB work correctly (retry logic validates)
• Suspend ByteArray: basic, nullable, large, concurrent