find: -printf: advance past a full char in advance_one to avoid a multibyte panic

[leeewee]

Fixes #730

advance_one read a full char via front() but then dropped it by slicing one byte (&self.string[1..]). When that char is multibyte (e.g. € after a % conversion, a \ escape, or a %A/%C/%T time directive), byte index 1 is not a char boundary, so the slice panicked:

$ mkdir d; touch d/a
$ find d -printf '%€\n'
thread 'main' panicked at src/find/matchers/printf.rs:152:35:
byte index 1 is not a char boundary; it is inside '€' (bytes 0..3) of `€`
$ echo $?
101

(also \€ and %A€ / %C€ / %T€)

This advances by the leading char's UTF-8 length (c.len_utf8()) instead of a fixed 1, so a multibyte char routes through the normal handling instead of aborting — the same multibyte-aware approach #723 applied to peek. Unrecognized escapes / invalid time specifiers keep their existing behavior (a multibyte char now behaves exactly like its ASCII equivalent, e.g. \€ like \q).

Added a regression test (test_parse_multibyte_char_after_directive).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.87%. Comparing base (795d29a) to head (13da7cc).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #731   +/-   ##
=======================================
  Coverage   91.86%   91.87%           
=======================================
  Files          35       35           
  Lines        7153     7160    +7     
  Branches      375      375           
=======================================
+ Hits         6571     6578    +7     
  Misses        437      437           
  Partials      145      145           

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[codspeed-hq]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

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_{Comparing leeewee:find-fix-printf-advance-multibyte (13da7cc) with main (795d29a)}