pub unsafe fn debug_validate_page(page: *const Page, where_: &str)Expand description
debug_checks invariant guard (our dmi equivalent): a page slot handed
to the hot paths must be a live SPAN START with self-consistent counters.
Catching a violated invariant here turns “mystery access violation” into
“this field was wrong, at this call site”.
§Safety
page must be a page slot the caller is already entitled to read.