refactor(receive): split file_save/incremental_check/delete_commit out

Pure structural split of src/shared/file_receive.c into focused translation
units behind the unchanged file_receive.h facade:

- file_save.c   : save-to-disk, special nodes, --delay-updates staging
- incremental_check.c : xattr/delta/basis/fuzzy receive + check state machine
- delete_commit.c : manifest receive + delete budget walkers
- file_receive.c : wire receive dispatch + deferred dir metadata

The shared receive_file_xattrs helper and MAX_FILE_DATA_SIZE are declared in
incremental_check.h.  file_save_to_disk_full_ex is decomposed into static
helpers (validation, special dispatch, dir/symlink creation, path resolution,
pre-write policies, data install) routed through one cleanup epilogue.

No behavior change.

src/shared/file_save.h

@@ -0,0 +1,40 @@
+#ifndef FILE_SAVE_H
+#define FILE_SAVE_H
+
+#include "config.h"
+#include "file_types.h"
+#include "format.h"
+#include <stdbool.h>
+
+/* Save-to-disk module: regular-file/symlink/hardlink/special install, xattr
+ * application, --fake-super and the --delay-updates staging path.  These
+ * declarations are re-exported by the file_receive.h facade. */
+
+/* Outcome of a single file_save_to_disk operation.  The receiver needs to
+   distinguish "written" from "skipped" so --remove-source-files can be told
+   which sources were actually stored. */
+typedef enum { FILE_SAVE_ERROR = 0, FILE_SAVE_WRITTEN = 1, FILE_SAVE_SKIPPED = 2 } FileSaveResult;
+
+bool file_special_rdev_valid(int32_t major, int32_t minor, mode_t mode);
+
+FileSaveResult file_save_to_disk_full(const char* root_directory, const File* file,
+                                      const Config* config);
+/* Protocol 2.28.0 variant: also reports through `created` (when non-NULL)
+ * whether the destination entry did not exist before this save, and through
+ * `created_dirs` how many parent directories the confined walk created, so the
+ * receiver can build rsync's `Number of created files` breakdown.  The plain
+ * file_save_to_disk_full() is this with both out-params NULL. */
+FileSaveResult file_save_to_disk_full_ex(const char* root_directory, const File* file,
+                                         const Config* config, bool* created,
+                                         unsigned* created_dirs);
+bool file_save_to_disk(const char* root_directory, const File* file, const Config* config);
+
+/* Protocol 2.28.0 receiver counter accumulator: fold one successfully saved
+ * entry into `stats`, adding its receiver-observed literal bytes and, when
+ * `created`, the matching created-by-type counter (regular file / symlink /
+ * special) plus `created_dirs` implicitly-created parent directories.
+ * Non-first hardlink siblings contribute no literal bytes. */
+void receiver_stats_note_saved(ReceiverStats* stats, const File* file, bool created,
+                               unsigned created_dirs);
+
+#endif