-printf: warn on unrecognized directives/escapes instead of dropping or erroring

[MsfPablo]

Summary

GNU findutils treats an unrecognized -printf directive/escape as a
warning and prints it literally, exiting 0. This diverged in two ways:

%X — the % was silently dropped

$ # GNU findutils 4.10.0
$ find d -printf '%€|\n'
find: warning: unrecognized format directive '%€'
%€|

$ # uutils, before this PR
$ find d -printf '%€|\n'
€|

\X — uutils errored instead of warning

$ # before this PR: hard error, exit 1
$ # after: warning + literal output, exit 0

Both cases now print a find: warning: ... message to stderr and emit
the directive/escape literally (with its %/\ prefix), matching GNU.

Along the way, fixed advance_one(): it sliced the remaining format
string at a fixed 1-byte offset, which panics on multibyte characters
(e.g. €, as in the issue's own repro) immediately after % or \.
Switched to char::len_utf8().

Test plan

• Added integration tests for both the multibyte (%€, \€) and
  the plain-ASCII unrecognized-directive case
• Updated the two existing unit tests (test_parse_escapes,
  test_parse_formatting) that asserted the old (incorrect) behavior
• cargo test — full suite green (226 unit + 61 integration tests)
• cargo fmt --check clean
• Manually verified output matches the GNU findutils transcript in the issue

Fixes #815

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.93%. Comparing base (1f19cdd) to head (bdea188).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #818   +/-   ##
=======================================
  Coverage   91.93%   91.93%           
=======================================
  Files          35       35           
  Lines        7251     7256    +5     
  Branches      378      378           
=======================================
+ Hits         6666     6671    +5     
  Misses        443      443           
  Partials      142      142           

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[github-actions]

Commit bdea188 has test result changes:

GNU findutils testsuite:

Test results comparison:
  Current:   TOTAL: 495 / PASSED: 418 / FAILED: 76 / SKIPPED: 1
  Reference: TOTAL: 495 / PASSED: 417 / FAILED: 77 / SKIPPED: 1

Changes from main branch:
  TOTAL: +0
  PASSED: +1
  FAILED: -1

Test improvements (1):
  + tests/find/printf_escapechars

bfs testsuite:

Test results comparison:
  Current:   TOTAL: 315 / PASSED: 267 / FAILED: 42 / SKIPPED: 6
  Reference: TOTAL: 312 / PASSED: 266 / FAILED: 40 / SKIPPED: 6

Changes from main branch:
  TOTAL: +3
  PASSED: +1
  FAILED: +2

New test failures (2):
  - gnu/files0_from_ok
  - gnu/okdir_path_empty

[codspeed-hq]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

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_{Comparing MsfPablo:printf-unrecognized-directive-warn (bdea188) with main (1f19cdd)}