fix(volume-rust): fix volume balance between Go and Rust servers (#8915)

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).

.github/workflows/rust-volume-server-tests.yml

@@ -93,7 +93,7 @@ jobs:
       - name: Build Go weed binary
         run: |
           cd weed
-          go build -o weed .
+          go build -tags 5BytesOffset -o weed .
           chmod +x weed
           ./weed version
 
@@ -169,7 +169,7 @@ jobs:
       - name: Build Go weed binary
         run: |
           cd weed
-          go build -o weed .
+          go build -tags 5BytesOffset -o weed .
           chmod +x weed
           ./weed version