Move the stats/progress reporting, scanner-preparation/scan helpers and
manifest/list/dry-run senders out of the ~3.9k-line client_send.c into
client_report.c, client_scan.c and client_manifest.c, sharing declarations
through the new internal client_send_internal.h. client_send.c keeps the
transfer orchestration and is now ~2.1k lines.
Decompose the monolithic send_files into static phase helpers
(send_files_prepare/_prepare_delete/_run/_finalize/_cleanup) driven by a
single SendFilesState; ownership, ordering and exit codes are unchanged.
No behavior change.
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