feat(easytier-go): accept data plane ABI v4

The core raised DATA_PLANE_ABI_VERSION to 4 in f26c2aa1 ("feat(wasi):
run EasyTier core on Cloudflare Workers and browsers") to advertise the
new guest exports easytier_data_plane_tcp_shutdown_write_submit and
easytier_data_plane_tcp_shutdown_write_result_take, which let a host
half-close guest TCP streams.

The Go host has no caller for half-close: net.Conn exposes only Close,
which already tears down both directions, so the v3 behavior is
preserved and no new plumbing is added. Without this bump, any artifact
rebuilt from current core source is rejected at instance creation with
"unsupported EasyTier data plane ABI version 4, want 3".

easytier-go/internal/coreabi/dataplane.go

@@ -12,7 +12,7 @@ import (
 )
 
 const (
-	DataPlaneABIVersion = 3
+	DataPlaneABIVersion = 4
 
 	DataPlaneCapability    uint64 = 1 << 0
 	DataPlaneTCPCapability uint64 = 1 << 1