s_write_intf_data_file uses wrong loop index for distance check

[sbryngelson]

Bug

In `src/post_process/m_data_output.fpp`, the subroutine `s_write_intf_data_file` has multiple issues in the distance-finding loop:

1. Uses stale loop variable `i` from a previous loop instead of the inner loop variable for Euclidean distance computation
2. Computes `euc_d` outside the inner search loop, so it never updates as the loop iterates
3. Uses `cycle` instead of `exit` when a match is found, continuing to search instead of stopping

File: `src/post_process/m_data_output.fpp`

Impact: The interface data file writer finds the wrong closest point for each interface location, producing incorrect interface data output.

Severity: High — interface data output contains wrong distance/location values.

[sbryngelson]

Fix: #1193