The 60k oneOf-object test had only one object node (the innermost), so the
pre-fix code called allocateClassName once — linear, never tripping the
timeout, so it did not cover the class-name Θ(depth²) it named. Give every
level an object branch (both oneOf arms are objects) so each level propagates
a one-segment-longer class name; the pre-fix rope slice is then Θ(depth²)
(~9.5s, past the 5s default) while the capped path stays linear. Also extract
the shared cap expression into capClassNameBase (used by allocateClassName and
childClassName). py-types.ts stays at 100% per-file coverage.