Update unfold-expression for cond final branch requirement

[tiye]

Using Calcit 0.13.13 on macOS arm64, a consumer pinned to the latest published Cirru/respo-cirru-editor@0.6.5 cannot preprocess after its dependencies are upgraded.

Reproduction from Cirru/cirru.org at latest main:

caps upgrade --all
caps --ci
cr compact.cirru --check-only

Actual result:

Failure: cond-requires-a-final-true-branch
  at cirru-editor.modifier.tree/unfold-expression @3.2.2.1.2.2.2.2.0

The stack originates in the published Cirru/respo-cirru-editor@0.6.5 module. The relevant cond needs an explicit final true branch for the current macro semantics.

Expected: the current tagged module preprocesses successfully under Calcit 0.13.13.

Environment: cr -v = 0.13.13; macOS arm64.