Two bugs prevented reliable volume balancing when a Rust volume server
is the copy target:
1. find_last_append_at_ns returned None for delete tombstones (Size==0
in dat header), falling back to file mtime truncated to seconds.
This caused the tail step to re-send needles from the last sub-second
window. Fix: change `needle_size <= 0` to `< 0` since Size==0 delete
needles still have a valid timestamp in their tail.
2. VolumeTailReceiver called read_body_v2 on delete needles, which have
no DataSize/Data/flags — only checksum+timestamp+padding after the
header. Fix: skip read_body_v2 when size == 0, reject negative sizes.
Also:
- Unify gRPC server bind: use TcpListener::bind before spawn for both
TLS and non-TLS paths, propagating bind errors at startup.
- Add mixed Go+Rust cluster test harness and integration tests covering
VolumeCopy in both directions, copy with deletes, and full balance
move with tail tombstone propagation and source deletion.
- Make FindOrBuildRustBinary configurable for default vs no-default
features (4-byte vs 5-byte offsets).
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