Adapt qed to the pyre::grid reimplementation

[aivazis]

Adapt qed to the pyre::grid reimplementation.

This tracks the pyre grid rewrite (pyre#211), which retires the per-cell-type grid
bindings and the typed-grid Python classes in favor of a single type-erased grid that
presents the buffer protocol. qed consumed the old model in three places — its C++ tile
generators, their bindings, and the Python readers — all of which are reworked here. The
numerical kernels in lib/qed are unchanged.

Grid bindings

The tile generators fall into two families:

• native and isce2 take a grid as their source. Each binding is replaced by one
  wrapper that receives a py::buffer (the erased grid, or anything else that speaks the
  buffer protocol), rebuilds a read-only typed grid over it with onGrid, and dispatches
  on the buffer's cell format to the existing kernel. The shared helper lives in
  ext/qed/grid.h/grid.icc: onGrid<dim, cellTs...> for the dispatch, and asGrid,
  asIndex, asShape for the conversions. This collapses the old per-cell-type overload
  piles to one wrapper per generator.

• nisar reads a tile out of an HDF5 dataset (using an on-disk datatype) and only uses
  the grid type at compile time for the tile geometry. Those wrappers keep the dataset and
  datatype and convert the geometry. The stack generators take a list of datasets; the
  masked variants take a second dataset for the mask; the BFPQ generators take the lookup
  table as a buffer, which asBFPQ rebuilds into the small heap grid the kernels expect.

Geometry arguments accept any Python iterable — tuple, list, or generator — so callers hand
in coordinates without wrapping them.

Readers

The readers no longer build Shape2D/Index2D/Canonical objects or per-type grid
factories. The three memory-mapped-file readers (native MemoryMap, the isce2 datasets)
lay a pyre.grid.map erased grid over the file; the rest pass their geometry as plain
sequences to the tile generators.

Datatypes

Each concrete datatype carries a cell attribute — the pyre memory cell name (float32,
complex64, …) — which is what pyre.grid wants when it lays a grid over memory. The
existing tag remains the class-name form used to build qed's own type-specialized binding
names (profileFloat).

Dependency

Requires pyre#211. Verified against the live client on NISAR GUNW products — the nisar real
and complex channels (including the masked coherence and the wrapped-interferogram phase)
render correctly.