Add FFM framework for replacing JNR-POSIX with Java Foreign Function & Memory API:

- FFMPosixInterface.java: Interface defining all POSIX functions
- FFMPosix.java: Factory with platform detection (Linux/macOS/Windows)
- FFMPosixLinux.java: Linux stub implementation
- FFMPosixMacOS.java: macOS stub (extends Linux)
- FFMPosixWindows.java: Windows implementation with Java/ProcessHandle fallbacks
- PosixLibrary.java: Add FFM integration points and feature flag docs

Feature flag: -Dperlonjava.ffm.enabled=true or PERLONJAVA_FFM_ENABLED=true

All methods throw UnsupportedOperationException in stubs, indicating
JNR-POSIX should be used until FFM implementations are complete.

See dev/design/ffm_migration.md for the full migration plan.

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Phase 5 now includes explicit steps for updating:
- build.gradle
- pom.xml
- CI workflows
- Documentation (README, QUICKSTART, installation guide, presentations)

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… API

- Implement FFM-based getuid, geteuid, getgid, getegid, getppid, umask, chmod, kill, isatty for Linux/macOS
- Add $< (REAL_UID) and $> (EFFECTIVE_UID) special variables with lazy evaluation
- Fix getppid JVM bytecode emission - was falling through to varargs handler
- Update Java version requirement to 22 (FFM finalized in Java 22)

The FFM implementation provides direct native calls without JNA overhead.
JNR-POSIX is still used as fallback for Windows and complex structs.

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- Add platform-specific struct stat layouts for Linux x86_64 and macOS
- Implement stat() and lstat() with proper errno capture
- Read struct fields with correct byte offsets for each platform
- Tested: values match native Perl exactly on macOS

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…, etc.)

- Add method handles for getpwnam, getpwuid, getpwent, setpwent, endpwent
- Define platform-specific struct passwd layouts for Linux and macOS
- Implement readPasswdEntry() to parse native passwd struct
- Add helper readCString() for reading null-terminated strings from native memory

Phase 3 (struct-based functions) is now complete with stat, lstat, and passwd functions.

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Phase 6 of FFM migration: complete removal of JNR-POSIX dependency.

Changes:
- Remove jnr-posix from build.gradle and libs.versions.toml
- Simplify PosixLibrary.java (FFM only, no JNR-POSIX fallback)
- Remove --sun-misc-unsafe-memory-access flag from jperl/jperl.bat
- Update documentation to remove sun.misc.Unsafe references

All native POSIX calls now use Java Foreign Function & Memory (FFM) API:
- NativeUtils: getuid, geteuid, getgid, getegid, getpid, getppid
- Stat: stat, lstat with platform-specific struct layouts
- KillOperator: kill signal handling
- Operator: chmod file permissions
- FileTestOperator, DebugHooks: isatty terminal detection
- UmaskOperator: umask file creation mask
- IOOperator: fcntl file descriptor control
- UtimeOperator: utimes file timestamp modification
- WaitpidOperator: waitpid process management
- ExtendedNativeUtils: getpwnam, getpwuid, getpwent, setpwent, endpwent
- POSIX: strerror error messages

Benefits:
- No more sun.misc.Unsafe deprecation warnings on Java 24+
- Reduced dependency footprint
- Uses standard Java API (FFM finalized in Java 22)

Tested: All 122 unit tests pass.

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The Dockerfile had an outdated JAR reference (3.0.0) that was missed
during previous version updates. Now that it matches the current version,
Configure.pl will correctly update it on future version changes.

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