[proof-new] Fix MACRO_RESOLUTION expansion for singleton clause corner case (#5850)

[proof-new] Fix MACRO_RESOLUTION expansion for singleton clause corner case (#5850)

Evaluating the proof infrastructure in SMT-LIB has uncovered a rare case (i.e., not previously in our regressions!!) in which the resulting clause from crowding literal elimination is a singleton. This commit makes the expansion code robust to that and adds an example of a problematic benchmark as a regression.

Also improves a bit tracing and some comments.