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

CHANGELOG.md

@@ -1,3 +1,24 @@
+## v0.65.0 — S3.1 offsite-tunnel client MTU 1420 → 1280 (resolve the CGNAT-smoke MTU open decision) (2026-07-04)
+
+One-constant fix closing `06 §4.3`'s OPEN DECISION. The 2026-07-04 CGNAT smoke test found the
+shipped interface MTU 1420 **silently black-holes bulk TCP** on any path below ~1480 B (mobile
+~1400, DS-Lite ~1452): the WG handshake and ping stay healthy (small packets) while the PBS TLS
+page — and, at S4, the backup itself — drops. "Looks green, loses backups." Must be safe before
+S4 flows bulk TCP over the tunnel.
+
+- **`internal/wgtunnel/manager.go`**: new `const clientMTU = 1280` (the RFC 8200 IPv6-minimum link
+  MTU — every path carries ≥1280; outer = 1280+60 v4 / +80 v6, fits every realistic path);
+  `renderConf` emits `MTU = %d` from it. **Fleet-wide, family-agnostic, permanent** — decouples
+  the fix from the v4/v6 endpoint-resolution question (§4.2). **Client-only by construction:** the
+  interface MTU caps box→PBS and the advertised MSS (=MTU−40) caps PBS→box, so the endpoint's `wg0`
+  is deliberately untouched (zero live-endpoint risk). Rejected: auto-probe / per-connection-type
+  (fragile moving part optimizing throughput, a non-metric here) and MSS-clamp (no forwarded flows).
+- **`internal/wgtunnel/manager_test.go`**: golden pins exact `MTU = 1280`; red-proofed (flip const
+  → 1420 fails the golden on the MTU line — non-vacuous).
+- **`internal/hub/report.go`**: stale "MTU 1420" comment → 1280 (still NOT a wire field).
+- No wire/JSON-golden change (MTU is a client-derived constant, never on the wire); no endpoint,
+  hub, controller, key, or desired-state change.
+
 ## v0.64.0 — S3 offsite WG tunnel: keygen + registration + agent-managed wg-quick@wg-felhom + escrow join (2026-07-04)
 
 The agent half of doc 06 §3.3 (felhom.eu S1/S2 built the endpoint + hub half). **DEFAULT OFF —