Keep references to local capsets in inferred types

[bracevac]

Fixes #26000

#25976 preserved ^{C} by marking closure parameters @declared in PostTyper, but only when the lambda is the direct RHS of a named definition.
It notably can't reach lambdas whose types end up in inferred types via arguments like Box([C^] => ...).

This fix adds a type-level rule to both cleanup passes: a retains set referencing a type-lambda binder that lives inside the inferred type itself is provably un-re-inferable, so it's kept and processed as a declared type

Have you relied on LLM-based tools in this contribution?

Yes, and I checked the output by guiding and reviewing it.

How was the solution tested?

New automated tests (including the issue's reproducer, if applicable)

& testing in the REPL via ./bin/replQ.