Language: default engine-thrown Throwable getCode() to 0 (#22945)

[PurHur]

Summary

• Engine-bridged Error / TypeError / DivisionByZeroError left ExceptionSupport::PROP_CODE unset in BuiltinExceptionSupport::materializeThrowable(), so Throwable::getCode() fatals on uninitialized Variable::$integer.
• Stamp default code = 0 to match php-src zend_throw_exception / zend_exception_get_props; fold materializeDivisionByZeroError() onto the shared path (net shrink of duplicated ObjectEntry setup).
• Compliance guard: test/compliance/cases/language/engine_throwable_getcode_default.phpt.
• Regenerated docs/bootstrap-inventory.md (OK 6517/6517).

Closes #22945

php-src reference

• Zend/zend_exceptions.c — default throwable code is 0
• PHP-in-PHP only (lib/VM/BuiltinExceptionSupport.php); no C runtime changes

Test plan

• Issue repro (host): Zend + php bin/vm.php /tmp/engine_getcode.php → all codes match
• vendor/bin/phpunit --filter engine_throwable_getcode_default test/compliance/VMTest.php → OK
• Related: exception_throw_code|exception_throw_caught|error_exception → OK
• php script/bootstrap-inventory.php --check → OK 6517/6517
• ./script/ci-fast.sh (in flight in Docker)

Verification transcript

=== ZEND ===
undef Error code=0
type TypeError code=0
div DivisionByZeroError code=0
user TypeError code=0
userErr Error code=7

=== VM (after fix) ===
undef Error code=0
type TypeError code=0
div DivisionByZeroError code=0
user TypeError code=0
userErr Error code=7

OK (1 test, 1 assertion)  # engine_throwable_getcode_default
OK 6517/6517              # bootstrap-inventory --check

C runtime: 0 lines added; PHP path only (duplicated DivisionByZero materializer removed).

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