Shrink CastHelper to thin dispatcher (#10244)

[PurHur]

Summary

• Shrinks lib/JIT/CastHelper.php from ~125 LOC to 42 LOC — thin dispatcher only
• Extracts native (array) / (object) / (unset) lowering into CastArrayNativeJit, CastObjectNativeJit, CastUnsetJit
• Refactors CastArrayRuntime bool branch to use shared JitVmHelperLink + CastJitHelper::boolYieldsEmptyArray (php-in-PHP SSOT per php-in-php: JIT CastHelper — route (array)/(object)/(unset) casts through VmCast PHP not ~495-line LLVM (#1492) #10046)

Closes #10244

php-src reference

• Zend/zend_operators.c — convert_to_array, cast_object
• VM SSOT: lib/VM/CastSupport.php, lib/VM/CastJitHelper.php

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
vendor/bin/phpunit --filter CastRuntimeShrink
vendor/bin/phpunit --filter EnumArrayCast
php bin/jit.php -r "var_export((array) new stdClass());"
php bin/jit.php -r "var_export((object) [\"a\"=>1]);"
php bin/jit.php -r "var_export((array) false);"
php bin/jit.php -r "var_export((array) true);"
'

Output:

OK (3 tests, 11 assertions)  # CastRuntimeShrinkTest
OK (3 tests, 3 assertions)   # EnumArrayCastTest
array ()
(object) array ('a' => 1,)
array ()
array (0 => true,)

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