numfmt panics (str char-boundary) on a number using a multibyte locale decimal separator before a multibyte char and a suffix

Under a locale whose decimal separator is multibyte (e.g. LC_ALL=ar_SA.UTF-8, where the separator is the Arabic ٫ U+066B, 2 bytes), numfmt aborts on an input like 1٫€K.

Steps to reproduce

$ LC_ALL=ar_SA.UTF-8 numfmt --from=si '1٫€K'
thread 'main' panicked at src/uu/numfmt/src/format.rs:70:20:
byte index 4 is not a char boundary; it is inside '€' (bytes 3..6) of `1٫€K`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Aborted (core dumped)
$ echo $?
134

The three match arms of find_valid_number_with_suffix each abort on a matching shape (all under LC_ALL=ar_SA.UTF-8):

GNU behavior

GNU rejects the malformed input and exit non-zero without crashing.

$ LC_ALL=ar_SA.UTF-8 /usr/bin/numfmt --from=si '1٫€K'
numfmt: invalid suffix in input: '1٫€K'
$ echo $?
2

Root cause

https://github.com/uutils/coreutils/blob/ebaf6e9c9f5442510c01c4b414abfe2a5c6accb2/src/uu/numfmt/src/format.rs#L53-L80

find_valid_number_with_suffix computes numeric_part.len() (a byte length) and then uses it both as a chars().skip(...) char count and as a byte-slice index &s[..=numeric_part.len()]. When the numeric part contains the multibyte separator, those two accountings desync: the char-skip lands on a later valid suffix, but the byte-slice cuts into the multibyte € mid-character, so the str slice panics (byte index N is not a char boundary) — panic=abort → SIGABRT, exit 134.

Start in src/uu/numfmt/src/format.rs at find_valid_number_with_suffix and reproduce the three multibyte-locale inputs from the issue. Trace the byte and character-boundary handling, then verify that malformed suffixes are rejected without a panic and that the relevant numfmt tests pass.