This PR fixes an `AppBuilder` exception in the `cbv` tactic when
simplifying projections whose projection function is dependent (closes
#12457).
Previously, `handleProj` unconditionally used `mkCongrArg` to prove `e.i
= e'.i` from `e = e'`, but `mkCongrArg` requires a non-dependent
function. For dependent projections (e.g., `fun x => x.2 : (x :
String.Slice) → x.1.Pos`), this would fail.
Now, `handleProj` first checks whether the projection function type is
non-dependent (a simple arrow). If so, it proceeds with `mkCongrArg` as
before. Otherwise, it falls back to:
1. Attempting to reduce the projection directly.
2. If reduction fails, using a heterogeneous congruence lemma
(`mkHCongr`) converted to an equality via `mkEqOfHEq`, provided the
original and rewritten struct are definitionally equal.
dae150a976