fix(placement): gate RDMA on nodeRdmaCtl.enabled at both endpoints (#2014)

## Summary

- Fixes a bug where `POST /place_instance` (and the dashboard UI) would
accept an MlxJaccl/RDMA instance spanning nodes whose
`nodeRdmaCtl.enabled` was `false`, because topology + placement
consulted Thunderbolt-derived RDMA edges without checking the per-node
`rdma_ctl` status.
- Three-layer fix: topology only emits `RDMAConnection` edges when both
endpoints have `nodeRdmaCtl.enabled = true`; flipping a node to disabled
immediately purges every RDMA edge touching it; `place_instance`
additionally rejects RDMA cycles containing any disabled or unobserved
node as a defense-in-depth check on the API/master path.

## Details

- `src/exo/shared/apply.py`
- `MacThunderboltConnections` case now filters out RDMA connections
whose source or sink lacks observed-and-enabled `rdma_ctl` status
(missing entry → treated as disabled).
- `RdmaCtlStatus` case now calls
`topology.remove_all_rdma_connections_touching(node_id)` when the node
reports disabled, so consumers don't have to wait for the next TB poll.
- `src/exo/shared/topology.py`
- New `Topology.remove_all_rdma_connections_touching(node_id)` removes
every RDMA edge incident to the node (incoming and outgoing) while
leaving socket edges intact.
- `src/exo/master/placement.py`
- `place_instance` accepts `node_rdma_ctl: Mapping[NodeId,
NodeRdmaCtlStatus] | None`. The `is_rdma_cycle` filter now also requires
`nodeRdmaCtl.enabled` for every node in the cycle. MlxJaccl placement
raises the existing "no RDMA-connected cycles available" error if no
qualifying cycle remains.
- `src/exo/api/main.py`, `src/exo/master/main.py`
  - Both placement entrypoints now pass `state.node_rdma_ctl` through.

## Tests

- `src/exo/shared/tests/test_apply/test_apply_rdma_gating.py` (new): six
unit tests covering enabled/disabled/missing combinations on apply, the
immediate-purge transition, and that purging RDMA edges leaves socket
edges untouched.
- `src/exo/master/tests/test_placement.py`: existing
`test_tensor_rdma_backend_connectivity_matrix` updated to pass
`node_rdma_ctl`. Two new tests assert MlxJaccl placement is rejected
when any cycle node is `enabled=false` or has no `rdma_ctl` entry.

## Test plan

- [x] `uv run basedpyright` — 0 errors
- [x] `uv run ruff check` — clean
- [x] `nix fmt`
- [x] `uv run pytest` — 429 passed, 1 skipped
- [ ] On a real mixed cluster (s15/s16 disabled, s17/s18 enabled),
confirm:
- [ ] `POST /place_instance` for an RDMA instance including s15 or s16
returns an error
  - [ ] An RDMA instance can still be placed across {s17, s18}
- [ ] `GET /state` shows no `sourceRdmaIface`/`sinkRdmaIface` on s15↔s16
connections
- [ ] Dashboard previews don't surface RDMA-spanning options that
include s15/s16

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>