Migrate from JNA to jnr-posix for native POSIX operations

[fglock]

Summary

• Replace JNA 5.18.1 (which uses deprecated sun.misc.Unsafe::staticFieldBase) with jnr-posix 3.1.19 for POSIX operations and pure Java APIs for Windows
• Eliminates both jna and jna-platform dependencies
• Net change: -1052 lines, +211 lines across 15 files

Key changes

• PosixLibrary: rewritten as thin wrapper around POSIXFactory.getNativePOSIX()
• WindowsLibrary: gutted (was entirely unused — no callers existed)
• NativeUtils: Windows kill uses ProcessHandle.destroy(), platform detection via System.getProperty("os.name") instead of JNA Platform class
• ExtendedNativeUtils: uses jnr.posix.Passwd instead of raw Pointer parsing
• Stat: uses jnr.posix.FileStat instead of Memory/Pointer parsing
• KillOperator: Windows uses ProcessHandle, POSIX uses jnr-posix kill() + errno()
• UtimeOperator: POSIX uses jnr-posix utimes(), Windows uses pure Java NIO
• WaitpidOperator: Windows uses Process/ProcessHandle, POSIX uses int[]
• Operator: Windows chmod uses Files.setAttribute("dos:readonly", ...)
• Removed 19 unused PosixLibrary method declarations (fork, execve, signal, etc.)
• Removed dead registerWindowsChildProcess code
• Added --enable-native-access=ALL-UNNAMED to jperl.bat

Test plan

• ./gradlew build passes (unit tests)
• make test passes
• make test-all — 45.2% pass rate consistent with pre-existing baseline, no regressions from migration
• Native-operation tests verified: stat_errors.t (638/638), io/fs.t (53/61), fork.t, waitpid.t, glob.t (17/18), kill0.t (6/9)
• Zero references to com.sun.jna remain in codebase
• Windows CI (pure Java fallbacks for kill/chmod/utime)

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