sdiff: implement the `|` marker for changed lines

src/side_diff.rs currently emits only <, > and blank gutters. Lines that exist in both files but differ are printed as a separate deletion and insertion instead of a single row with |.

The | is produced at print time, by pairing positionally inside a contiguous change hunk. For a run of N deletions and M insertions, the first min(N, M) are printed as pairs with |, and the leftovers keep < or >. The pairing is positional, so the two lines of a | row need nothing in common:

$ diff -y --width=40 L R      # L: a,b1,b2,b3,c   R: a,X,c
a			a
b1		   |	X
b2		   <
b3		   <
c			c

So the change is confined to the consumption of the current engine diff's. Buffer the run of Left/Right results, and on the next Both (or at EOF) flush it with the pairing above.

Start in src/side_diff.rs and trace how the current engine diff’s Left, Right, and Both results are consumed. Verify the change with the provided diff -y example, including a hunk with unequal deletion and insertion counts. Done means positional pairs in each contiguous change hunk print with |, while leftovers retain < or > and unchanged lines remain unaffected.