core: introduce the file_ops platform seam (ADR 0020 Phase 0)

Pure refactor, zero behaviour change. posix_fallocate/ftruncate,
posix_fadvise(DONTNEED), and fdatasync move out of sparse_file.cpp and
meta/veloxpart.cpp into core/src/io/platform/linux/file_ops.cpp behind
a new seam (core/src/io/platform/file_ops.hpp). Neither caller names a
Linux-only syscall anymore, and no #ifdef exists outside platform/linux/.
Clears the way for the PORT lane's macOS backend.

Full suite: 44/44 green (build/dev).

Co-Authored-By: Claude Sonnet 5 <[email protected]>

core/CMakeLists.txt

@@ -20,6 +20,7 @@ add_library(veloxcore STATIC
     src/net/probe.cpp
     src/io/sparse_file.cpp
     src/io/write_buffer.cpp
+    src/io/platform/linux/file_ops.cpp
     src/meta/veloxpart.cpp
     src/segment/segmenter.cpp
     src/segment/budget.cpp