Record every cabled pair in a jaccl-ring hostfile

[erwinzhang7]

configure_jaccl_ring writes None for every pair that is not a ring neighbour, including
pairs that are physically cabled:

for other in ring:
    if other not in (peer_left, peer_right):
        rdmas.append(None)

configure_jaccl a few lines above emits the full N x N matrix, so connectivity survives in a
mesh hostfile and is discarded in a ring one, before anything in C++ sees it.

The information is already available. IPConfigurator keys on every connected Thunderbolt port
and assigns addresses per cable, and its setup() brings up an interface for each pair it
holds, so the machines are cabled and configured for the whole topology while the hostfile
describes a subset of it.

Nothing changes how the group runs. MLX_JACCL_RING selects the ring, not the shape of
the matrix, and launch_jaccl only requires the matrix to be square with a null diagonal. Both
still hold, ring neighbours keep the width the ring was built with, and a hostfile for a
topology with no extra cables is byte-identical to before.

What it buys is that a link is no longer lost. A subgroup can only contain members that are
directly connected, so the pairs dropped here are the ones Group.split() would need, and the
hostfile is everything the runtime is given. Related to #3205.

The matrix construction moves into jaccl_ring_devices(ring, count, ips), a pure function over
a device map, so it is testable without a cluster. Five tests cover a plain ring, a ring with a
chord, the square-and-null-diagonal contract launch_jaccl enforces, ring ordering, and
multiple cables between neighbours.