Clear class_proxy.class_binding after its one-time use - #50

What

class_proxy.class_binding captures the class body's Binding via TracePoint :end, used once to eval default/type-expression values (e.g. the bare
String in def method(var: String)) against the class's own lexical scope.
I checked every consumer across low_type, lowkey, and every gem that
depends on either -- nothing reads it after that one evaluation.

Leaving it set matters for more than tidiness: a Binding can never be
made Ractor-shareable, even frozen:

b = binding
b.freeze          # => succeeds, b.frozen? == true
Ractor.make_shareable(b)
# => Ractor::Error: can not make shareable object for #<Binding:0x...>

So a live Binding sitting on class_proxy -- which lives inside Lowkey.keys,
the file/class/method-proxy registry every LowType-included class is stored
in -- permanently blocks Ractor.make_shareable(Lowkey.keys), i.e. sharing
that whole registry across Ractors, regardless of anything else.

Fix

Set class_proxy.class_binding = nil right after the one evaluation that
needs it. Confirmed via Adapter::Loader.load (the only other thing that runs
in the same TracePoint callback) that nothing downstream touches it either.

Testing

• New spec: spec/units/class_binding_spec.rb -- loads a real fixture class
  and asserts class_binding is nil afterward.
• Full suite: 142 examples, 0 failures.
• Rubocop clean (one pre-existing, unrelated Metrics/AbcSize offense on
  .included, present on main before this change).

Second of three PRs from a look at what it'd take to make Raindeer's stack
Ractor-safe (see #49 for the first). This one is necessary but not
sufficient -- Lowkey itself still needs a way to actually call
Ractor.make_shareable on its registry; that's a separate PR against the
lowkey repo.

🤖 Generated with Claude Code