Add 4 KiB logical block footprint tracking to stats

[seks99x]

Fixes #1066
Original feature request by @birdie-github

Currently, rsync --stats reports Literal data, which represents the unmatched file-update data sent over the network. However, network payload is often completely disconnected from the actual disk I/O footprint on the receiving end. Depending on the transfer flags (e.g., default temp-file rebuilding vs. --inplace), a tiny network payload can result in massive disk writes, or vice versa. System administrators currently have no native visibility into the true I/O cost of a sync.

This PR introduces a new metric to the --stats output: Number of modified 4K blocks. It tracks exactly how many 4,096-byte boundaries are written to the receiver's disk, completely decoupling network delta payload from disk I/O footprint.
O(1) Zero-Overhead Implementation
To ensure tracking does not bottleneck the receiver's high-speed I/O loop, the block calculation operates entirely in O(1) time with zero system calls and no loop iterations.
Stateful Tracking: It uses a static state machine (last_tracked_fd and last_touched_blk) to remember the last logical block written. Because the receiver strictly writes sequentially, we can calculate block deltas using purely mathematical boundaries.
Math over Memory: For each write, it calculates the block boundaries using (offset % 4096) and len. It diffs this against the last known state and instantly updates the global counter.

Real-World Examples (Why this matters)
Here is a demonstration of how network data and disk I/O diverge, using a 4MB file where we inject a single byte at 10 different 4K block boundaries:

Case 1: Delta transfer with --inplace

# 1. Restore the base file to exactly match the scattered destination
cp dest_scattered.bin base.bin

# 2. Inject a single byte at 10 different 4K block boundaries
python3 -c 'f=open("base.bin", "r+b"); [(f.seek(i*4096), f.write(b"\xFF")) for i in range(1,11)]; f.close()'

# 3. Sync it
../rsync -a --stats --inplace -I --no-whole-file base.bin dest_scattered.bin | egrep "Literal data|touched"
Literal data: 20,480 bytes
Number of modified 4K blocks: 10

Context: Rsync sends a small amount of literal data over the network to patch the file. Because --inplace is used, the receiver seeks and updates only the 10 specific blocks on disk. Network and Disk I/O are both low.
Case 2: Delta transfer without --inplace (Default temp-file behavior)

# 1. Restore the base file to exactly match the scattered destination
cp dest_scattered.bin base.bin

# 2. Inject a single byte at 10 different 4K block boundaries
python3 -c 'f=open("base.bin", "r+b"); [(f.seek(i*4096), f.write(b"\xFF")) for i in range(1,11)]; f.close()'

# 3. Sync it
../rsync -a --stats -I --no-whole-file base.bin dest_scattered.bin | egrep "Literal data|touched" 

Literal data: 0 bytes
Number of modified 4K blocks: 1,024

Context: Rsync perfectly matches the unchanged data locally. The network payload is effectively zero. However, because rsync reconstructs the file into a new hidden temp file before moving it into place, the receiver's disk is forced to rewrite the entire 4MB file (1,024 blocks).

[steadytao]

Looks quite good to me overall, thank you. Note that CI failures are not because of your changes. Ubuntu is fixed on master, Cygwin will soon be the same so you can rebase then.

[seks99x]

@steadytao ubuntu workflow is the only one forced running rsync with --protocol=30 which is failing the tests.

[steadytao]

Will fix Ubuntu.

[steadytao]

git diff --check reports trailing whitespace --

The C indentation is inconsistent, the sparse test still describes “physical” blocks/disk writes and the test file lacks a final newline. Please use the logical-region terminology consistently throughout the test as well.

[seks99x]

Yes thanks! the test name is changed also so you need to reconfigure the skiplist.

[seks99x]

I fixed the trailing spaces and reverted SUBPROTOCOL_VERSION back to 0. Sorry mate I forced push so I hope it didn't clear your skip list config. ( :( Didn't notice )

[steadytao]

I fixed the trailing spaces and reverted SUBPROTOCOL_VERSION back to 0. Sorry mate I forced push so I hope it didn't clear your skip list config. ( :( Didn't notice )

It happens. Make sure to force with lease next time :D