[PurHur]

Summary

• mb_substr(), mb_strimwidth(), mb_strcut(), and mb_strpos() int operands now use frame-aware guards (parseIntBuiltinArgForFrame) via shared VmMbstring helpers.
• Under declare(strict_types=1), float offsets/lengths raise TypeError; coercive call sites still truncate like Zend.
• php-src reference: ext/mbstring/mbstring.c.

Closes #13849

Test plan

./script/docker-exec.sh -- php test/repro/maintainer_gap_mb_substr_float_offset.php
# exit 0

./script/docker-exec.sh -- php bin/vm.php test/repro/maintainer_gap_mb_substr_float_offset.php
# exit 0

./script/docker-exec.sh -- bash -lc 'cd /compiler && vendor/bin/phpunit test/compliance/MbFloatIntStrictVMTest.php'
# OK (1 test)

./script/docker-exec.sh -- bash -lc 'cd /compiler && php bin/compile.php -l test/repro/maintainer_gap_mb_substr_float_offset.php'
# AOT lint OK

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