Add PerLayerTransferContext: the per-request coordinator for overlapped per-layer
KV transfer. submit_layer(layer_id, event) waits the layer's CUDA write event (on
a background thread, never the compute stream) before async-submitting that
layer's RDMA via the G1 path, so the transfer never reads a layer before its
write kernel finished — the core correctness invariant for the forward overlap.
finish() waits all accumulated batch_ids; idempotent per layer; fails closed.
Unit-tested (test_cp_per_layer_transfer.py, 6 cases): event-wait-before-submit
ordering, idempotency, empty-layer skip, finish-waits-all, submit-failure stop,
wait-failure propagation. The scheduler/notifier wiring (A2-wiring + A3) builds
on this.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
leavelet
e0d47bfc41