Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@crates/capi/src/setobject.rs`:
- Around line 45-50: PySet_Add currently downcasts the `set` argument only to
`PySet`, rejecting valid `PyFrozenSet` inputs; update `PySet_Add` (in
`crates/capi/src/setobject.rs`) to accept either a `PySet` or a `PyFrozenSet`
(and subtypes) by detecting the concrete type after `with_vm` unwrap and calling
the appropriate mutator: call `PySet::add` for `PySet` and invoke the VM-side
frozenset-initialization/mutation path for `PyFrozenSet` (the same internal
routine used to populate brand-new frozensets) so that `set.add(key, vm)`
semantics are preserved; ensure you clear/reset any cached hash on the
`PyFrozenSet` after mutation so subsequent hash lookups are correct and preserve
existing `with_vm` error propagation.
- Around line 82-90: Replace the current two-step membership + removal in
PySet_Discard with a single discard call that returns whether an item was
removed: add a bool-returning helper method on PySet (e.g.,
PySet::discard_returning_bool) that wraps the existing
PySetInner::discard(...)->PyResult<bool>, then update PySet_Discard to call that
helper once and return its boolean result as c_int; keep using the existing
try_downcast_ref::<PySet> and the same key handling but remove the prior
__contains__ probe so lookup+removal happen in one operation.