Inherited method in a two-level ClassExprFresh chain sees undefined for a captured local (capture forwarding, not instanceof)

[proggeramlug]

Found while fixing #10624 (PR #10640) and confirmed unrelated to instanceof — filing separately so it is not
lost in that PR's body.

Shape: a two-level ClassExprFresh chain, where an inherited method closes over a captured local, returns
undefined instead of the captured value.

This is capture forwarding through a dynamically-evaluated class chain, not a prototype or instanceof question.

Likely neighbours — check these first

• PR fix(hir): forward inherited captures through a locally-shadowed class parent #10628 (Part of #10486) — forwards inherited captures through a locally-shadowed class parent, and
  explicitly does not close Inherited method of a capture-bearing class expression sees undefined captures when called on an instance of a capture-bearing class extends subclass without a constructor #10486: a subclass with an empty body, or one whose base is a class declaration,
  still fails, because codegen/mod.rs's packed-field parent_chain walk keys off Class.extends_name which that
  fix deliberately doesn't write back. A two-level chain is exactly the shape that walk gets wrong.
• PR fix(hir): capture class member enclosing vars by reference, not snapshot #10615 (Fixes #10485/#10489) — replaced the Let-counting heuristic in shared_mutable_capture.rs with
  a DeclCensus, because a var is declared twice and so never satisfied "exactly one declaration".

If this turns out to be the remaining half of #10486 rather than a distinct defect, say so on both issues and
close whichever is the duplicate.