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.

CMakeLists.txt

@@ -99,17 +99,20 @@ set(SHARED_SRCS
 	src/shared/daemon_limits.c
 	src/shared/data.c
 	src/shared/delay_updates.c
+	src/shared/delete_commit.c
 	src/shared/delete_plan.c
 	src/shared/delta.c
 	src/shared/file.c
 	src/shared/file_list.c
 	src/shared/file_receive.c
+	src/shared/file_save.c
 	src/shared/file_send.c
 	src/shared/file_store.c
 	src/shared/filter.c
 	src/shared/format.c
 	src/shared/hardlink.c
 	src/shared/identity.c
+	src/shared/incremental_check.c
 	src/shared/log.c
 	src/shared/metadata.c
 	src/shared/motd.c