lower `enzymexla.linalg.qr` to `enzymexla.lapack.geqrf/geqrt`

`enzymexla.linalg.qr` should serve just as a high-level op that dispatches (i.e. lowers) to different QR algorithms and fulfills that returns the Q and R matrices (either in their full or thin versions). it also has its own derivative rule that we should write.

Start by locating enzymexla.linalg.qr and the existing enzymexla.lapack.geqrf/geqrt operations. Define the dispatch behavior for full and thin Q/R results, then implement the QR derivative rule; the issue is done when both lowering and differentiation are supported.

Written by the indexing model from the issue text.