Stdlib: null operand coercion parity batch (#18369–#18378)

[PurHur]

Summary

• Add InternalStrictArg::resolveCoercibleStringArg() for php-src Z_PARAM_STR parity: null coerces to '' outside declare(strict_types=1), still TypeError under strict call sites.
• Fix 7 null-coercion gaps across datetime, PCRE, array sort, mktime, touch/unserialize, and scandir/exec/popen paths on VM + JIT lowering.
• Add compliance .phpt guards + maintainer repro scripts for each issue.

php-src reference

• ext/standard/datetime.c — php_date, php_gmdate, php_strtotime, php_mktime
• ext/pcre/php_pcre.c — subject string coercion
• ext/standard/array.c — sort flags default
• ext/standard/filestat.c, ext/standard/var.c — touch/unserialize empty input
• ext/standard/dir.c, ext/standard/exec.c, ext/standard/proc_open.c — empty path/command

PHP implementation

• lib/VM/InternalStrictArg.php — shared resolveCoercibleStringArg()
• ext/standard/{strtotime,date,gmdate,preg_match,sort_,rsort_,mktime,gmmktime,touch_,unserialize,scandir,exec,popen}.php + matching JIT helpers

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/vm.php test/repro/maintainer_gap_strtotime_null.php'
# false

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/vm.php test/repro/maintainer_gap_mktime_null_coerce.php'
# mktime=int:yes
# gmmktime=int:yes

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/vm.php test/repro/maintainer_gap_touch_unserialize_null.php'
# touch=false
# unserialize=false

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter "strtotime_null_datetime|preg_match_null_subject|sort_null_flags|mktime_null_coerce|touch_unserialize_null|scandir_exec_null|date_gmdate_arg_parity"'
# 16/17 pass (VM+JIT); AOT date_gmdate_arg_parity pre-existing red on master (compile error exit 2 → segfault exit 139 on this branch — tracked separately)

Closes #18378
Closes #18377
Closes #18376
Closes #18375
Closes #18370
Closes #18369
Closes #18371

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