wgtunnel: client MTU 1420 → 1280 (S3.1, resolve §4.3 open decision)

The 2026-07-04 CGNAT smoke test found MTU 1420 silently black-holes bulk TCP
on sub-~1480 paths (mobile ~1400, DS-Lite ~1452): handshake+ping stay healthy,
PBS TLS page (and at S4 the backup itself) drops. Set a fleet-wide, permanent,
family-agnostic client MTU of 1280 (RFC 8200 IPv6-minimum floor; outer 1340 v4
/ 1360 v6 fits every realistic path).

Client-only by construction — interface MTU caps box→PBS, advertised MSS caps
PBS→box; the endpoint's wg0 is untouched (zero live-endpoint risk). New const
clientMTU=1280 as the single home; golden pins exact "MTU = 1280" (red-proofed
against a 1420 flip). Stale report.go comment updated. No wire/JSON change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PSK5g6qYLknKj8u3QAFEr6

internal/wgtunnel/manager_test.go

@@ -130,6 +130,9 @@ func localPub(t *testing.T, m *Manager) string {
 	return pub
 }
 
+// TestRenderConf_Golden pins the EXACT conf bytes, including "MTU = 1280" (the IPv6-minimum
+// floor, doc 06 §4.3). Red-proofed 2026-07-04: flipping clientMTU back to 1420 fails this test
+// on the MTU line mismatch — the golden is non-vacuous, not a "contains MTU" check.
 func TestRenderConf_Golden(t *testing.T) {
 	block := testBlock("ignored")
 	conf, err := renderConf(block, vectorPrivB64)
@@ -140,7 +143,7 @@ func TestRenderConf_Golden(t *testing.T) {
 [Interface]
 PrivateKey = ` + vectorPrivB64 + `
 Address = 10.77.0.2/32
-MTU = 1420
+MTU = 1280
 
 [Peer]
 PublicKey = CQkJCQkJCQkJCQkJCQkJCQkJCQkJCQkJCQkJCQkJCQk=