dist_send NIF wrapper omits native_context for error reporting

[ausimian]

Severity: low

Emily.Native.dist_send/4 calls await/1 without a native_context map (lib/emily/native.ex:145), unlike the generated collectives and every other op. A failure (e.g. invalid dst) carries no op/input context in its error.

Fix: pass native_context(:dist_send, w, x: x) (matching the collective loop above).

Found in code review of the distributed-collectives scaffold (feature/distributed-mlx-scaffold).