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.
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