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/incremental_check.h

@@ -0,0 +1,41 @@
+#ifndef INCREMENTAL_CHECK_H
+#define INCREMENTAL_CHECK_H
+
+#include "config.h"
+#include "file_types.h"
+#include "protocol.h"
+#include <stdbool.h>
+
+/* Incremental-check module: the per-file STATUS_CHECK state machine, the
+ * incremental delta / alternate-basis / fuzzy matching helpers and the shared
+ * xattr receive helper.  These declarations are re-exported by the
+ * file_receive.h facade. */
+
+/* Whole-file payload bound shared by the plain receive path and the
+ * incremental check paths. */
+#define MAX_FILE_DATA_SIZE MAX_RECEIVE_WHOLE_FILE_SIZE
+
+/* Receive a file's xattr block (when the config enables xattr transport) and
+ * attach it to `file`.  Returns false on a malformed/oversized frame. */
+bool receive_file_xattrs(File* file, int fd, const Config* config);
+
+File* receive_incremental_check(int fd, const Config* config, bool* skipped);
+/* Extended variant used by the receiver.  `would_transfer` (may be NULL) is set
+ * true only on the server-contacting --dry-run path when the file is not up to
+ * date: the receiver has already sent STATUS_DRY_RUN_TRANSFER and returns NULL
+ * without storing anything.  On that path `*skipped` is true for an up-to-date
+ * (STATUS_OK) file and both flags are false for a genuine error. */
+File* receive_incremental_check_ex(int fd, const Config* config, bool* skipped,
+                                   bool* would_transfer);
+
+/* Testable basis quick-check / verification policy.  file_basis_quick_match is
+ * rsync's metadata quick-check for a basis candidate (equal size is required
+ * separately by the caller; this adds the --size-only / mtime / --modify-window
+ * leg).  file_basis_content_required reports whether a hit must ALSO be
+ * confirmed by a whole-file content digest (--verify-basis; false is the
+ * default rsync-parity behavior). */
+bool file_basis_quick_match(const Config* config, const struct stat* st, time_t check_mtime,
+                            long check_mtime_nsec);
+bool file_basis_content_required(const Config* config);
+
+#endif