Fix mbregex retry-limit failures in Multibyte::trim

[vanderlee]

Closes #27.

Long internal whitespace runs cause the trailing \s*$ alternative in Multibyte::trim() to backtrack and retry from successive positions. This can exhaust mbregex's retry limit, emit a warning, and return false instead of the original text.

Use absolute string anchors, possessive whitespace quantifiers, and a negative whitespace lookbehind for the trailing alternative. Each whitespace run is consumed without backtracking and cannot be retried from inside the run. This retains mbregex's encoding and Unicode whitespace support without increasing retry limits or suppressing errors.

Regression coverage includes the reporter's exact 801/1348-space input, the same input with surrounding whitespace, a 20,000-space internal run, empty and whitespace-only strings, CR/LF/tab boundaries, internal newlines, non-breaking spaces, ideographic spaces, and multibyte text. Tests explicitly set and restore UTF-8 for the fixtures and lower mbstring.regex_retry_limit to 1000 when available; PHPUnit restores the limit afterward. Older PHP versions still run the same input cases.

Validation:

• Before the fix, the trim tests produce three retry-limit-in-match over errors with the limit set to 1000, including the reported input. Current local runtimes did not fail on that input at their default limit.
• composer test, PHP 8.2.10 / PHPUnit 9.6.37: 47 tests, 133 assertions passed.
• Full PHPUnit suite in an isolated Linux PHP 8.3.4 container: 47 tests, 133 assertions passed.
• All seven CI checks passed: PHP 5.4, 5.6, 7.4, 8.0, 8.4, 8.5, and 8.6.
• composer validate --strict and git diff --check passed.
• PHPUnit 9 reports the existing phpunit.xml legacy <filter> configuration warning, also present before this change.