Restore macOS immutable file-flag support with a minimal Java 25 FFM handler

[Copilot]

PR eclipse-platform#2925 moved macOS local-file metadata updates to PosixHandler, which preserved POSIX permissions but dropped BSD st_flags handling. That regressed EFS.ATTRIBUTE_IMMUTABLE on macOS, so ATTRIBUTE_READ_ONLY no longer round-tripped through cache copies as expected by the existing resources tests.

• What changes on macOS

  • Add a small macOS-only handler that keeps normal metadata work in PosixHandler and restores only the missing BSD file-flag behavior.
  • Advertise EFS.ATTRIBUTE_IMMUTABLE only when the macOS FFM path is available.
  • Keep non-macOS behavior unchanged.

• FFM-based BSD flag access

  • Add MacFileFlags, a Java 25 FFM helper that calls macOS libc directly.
  • Use lstat to read st_flags without following symlinks.
  • Use chflags to update flags while preserving unrelated bits.
  • Treat both UF_IMMUTABLE and SF_IMMUTABLE as immutable when fetching attributes.
  • Only set/clear the user-settable UF_IMMUTABLE bit on write.

• Attribute update ordering

  • Preserve the native implementation’s ordering:
    1. clear user immutable when needed,
    2. apply POSIX permission changes through PosixHandler,
    3. restore/set immutable afterward.
  • Avoid overwriting unrelated BSD flags introduced or preserved across the POSIX update.

• Handler selection

  • Update LocalFileNativesManager to select the macOS immutable-aware handler only for macOS POSIX filesystems where the FFM bindings are usable.
  • Leave Linux fast-path natives and all other handlers untouched.

• Focused regression coverage

  • Keep TestBug323833’s small-file and large-file cache-path assertions intact.
  • Strengthen macOS coverage with direct fetch-after-put assertions and explicit immutable/read-only round-trip checks in Bug_329836.

• Why NIO alone is not enough

  • Standard Java NIO exposes POSIX permissions, but not macOS BSD st_flags.
  • This change uses FFM only for the missing BSD-file-flags portion rather than reintroducing a native macOS fragment.

IFileInfo info = fileStore.fetchInfo();
info.setAttribute(EFS.ATTRIBUTE_READ_ONLY, true);
fileStore.putInfo(info, EFS.SET_ATTRIBUTES, monitor);

IFileInfo updated = fileStore.fetchInfo();
assertTrue(updated.getAttribute(EFS.ATTRIBUTE_READ_ONLY));
assertTrue(updated.getAttribute(EFS.ATTRIBUTE_IMMUTABLE));