Prefer libfuse3 for pfsfuse with libfuse fallback compatibility

[Copilot]

pfsfuse currently depends on fuse in Meson, which breaks builds in environments standardizing on fuse3 (e.g., nixpkgs). This change updates dependency resolution and adapter compatibility so pfsfuse builds against fuse3 while retaining support for legacy fuse.

• Build dependency resolution

  • Prefer dependency('fuse3') on non-Windows builds.
  • Fall back to dependency('fuse') when fuse3 is unavailable.
  • Propagate a compile-time switch to select the correct API path.

• FUSE2/FUSE3 adapter compatibility

  • Add conditional header/version selection:
    • fuse3: FUSE_USE_VERSION 30, <fuse3/fuse.h>
    • fallback: existing fuse2 include/version path
  • Update operation callback signatures where FUSE3 differs (init, readdir, getattr, rename, chmod, utimens).
  • Wire truncate op by API version (.truncate for FUSE3, .ftruncate for FUSE2).
  • Handle FUSE3 rename flags safely by rejecting unsupported non-zero flags.

• CI and docs alignment

  • Update Ubuntu CI dependency install to fuse3/libfuse3-dev.
  • Update README Linux guidance to recommend fuse3 with fuse fallback on older systems.

libfuse_dep = dependency('fuse3', required: false)
if not libfuse_dep.found()
  libfuse_dep = dependency('fuse')
endif