Pruning a Simplicity case expression
The definition of cond is

cond thn els = match (drop els) (drop thn)


Expanding this in your definition of full yields

full = match (drop unit) (drop u)


The correct pruned version should be

pruned = assertl (drop unit) (commitmentRoot (drop u))


which has commitment root that matches:

let full = cond unit unit in (flip Numeric.showHex "".integerHash256.commitmentRoot) (full :: CommitmentRoot (Bit, ()) ())
"dded2b0938bb9f58f20324ab1f699e7006ffd8e9251b3ddd0f551de1d66fe119"

let pruned = assertl (Simplicity.Term.Core.drop unit) (commitmentRoot (Simplicity.Term.Core.drop unit :: CommitmentRoot ((),()) ())) in (flip Numeric.showHex "".integerHash256.commitmentRoot) (pruned :: CommitmentRoot (Bit, ()) ())
"dded2b0938bb9f58f20324ab1f699e7006ffd8e9251b3ddd0f551de1d66fe119"


Better library support for pruning expressions is something that will be developed.