Replace the IS_PROP_CPP_REINITABLE property flag and IS_OBJ_CTOR_CALLED
object flag with the existing IS_PROP_REINITABLE flag, following the
same try/finally pattern already used by __clone().

How it works:
- On CPP implicit initialization, if the current execute frame's scope
  matches prop_info->ce (no stack walk needed), set IS_PROP_REINITABLE
  instead of clearing all property flags. This opens a one-shot
  reassignment window.
- zend_leave_helper clears IS_PROP_REINITABLE from all PROMOTED|READONLY
  properties of the constructor's scope on exit, gated on
  ZEND_CALL_HAS_THIS|ZEND_ACC_CTOR (covers both `new Foo()` and
  `parent::__construct()` calls, not just ZEND_CALL_RELEASE_THIS).
- zend_is_readonly_property_modifiable() simplified to a single
  IS_PROP_REINITABLE check; zend_is_in_declaring_constructor() and the
  call-stack walk helper removed entirely.

A repeated __construct() call cannot bypass readonly because the
property is already past IS_PROP_UNINIT, so IS_PROP_REINITABLE is never
re-set; no object-level tracking flag is needed.

This saves two flag bits (property and object extra_flags) and removes
~30 lines of stack-walk code.