SC-10: progress reporting, transfer stats, verbose output

[iampryce]

--progress via goroutine+buffered-channel reporter, non-blocking sends so a slow/absent consumer never stalls a transfer. Progress shown for the disk-write phase (chunked writes over 256KiB), not network transfer

• a disclosed scope boundary, since ApplyDelta's delta frames arrive atomically per file with no observable partial-transfer point in the current architecture. Output format matches real rsync's man-page examples line-for-line.

--stats end-of-sync summary matching real rsync's field names/ structure, with the speedup ratio formula verified against rsync's actual main.c source, not just man-page prose. -v confirmed already correct from SC-11, no reimplementation needed.

Threaded through all transports (local, SSH, daemon GET/PUT) reusing SC-11's exact dryRun/itemize pattern.

Three bugs found in self-review:

• Reporting() broadening leaked verbose-style lines when only --stats/--progress was requested without -v/-i
• formatDuration omitted the hours component at zero, contradicting the man page's own examples
• A new empty file was miscounted as unchanged in stats, since bytes.Equal(nil, nil) == true - fixed with a separate transferred check distinct from contentChanged

Clean on native Windows and cross-compiled Linux.