[CMGS]

The guest kernel enables USO only when both USO4 and USO6 are offered. The host's hello offered USO4 alone, so the guest's tun0 kept tx-udp-segmentation off. Every frame the guest sent therefore carried a single datagram, and the host's UDP GSO path never ran.

Change

• OffloadUSO6 = 0x40 is added to segmentable.
• The host still drops an IPv6 frame, as it drops any non-IPv4 packet.
• docs/advanced-network.md: the hello table and the offload paragraph now list USO6.
• TestRouterAnnouncesRewritesAndPassesOnlyReplies asserts that the host hello carries both TSO4 and the USO pair. The test fails against the old segmentable.

Evidence (hardware, base e38c762; 8 runs per arm in ABBABAAB order)

Guest sender	Before	After	Change
UDP_SEGMENT → server netns	972 Mbit/s	2438 Mbit/s	+151%, p=0.0008
UDP_SEGMENT → host veth	878 Mbit/s	1897 Mbit/s	+116%, p=0.0008
Plain UDP			+0.8% / +0.7%

• With the change, a guest frame carries 42 datagrams.
• The two arms' ranges do not overlap.
• No frames were dropped across the 64 runs.

Host to guest is unaffected: the host merges only TCP toward the guest.

Gates

• make go-lint: 10× 0 issues.
• asl -forwarder=false ./...: clean on both GOOS.
• go test -race ./tunnet/: ok.