Raise the overlay MTU to 1280: below that Linux disables IPv6

The setup recipe failed with a missing sysctl directory and "RTNETLINK
answers: Invalid argument". The cause was the default MTU of 1100.

IPv6 requires a minimum MTU of 1280 (RFC 8200) and Linux enforces it by
tearing IPv6 down on any interface below it: the per-device
/proc/sys/net/ipv6/conf entries disappear and an address can no longer be
assigned. Evidence on the test host: every interface at 1280 or above has
an IPv6 conf directory, every interface below it (1230, 1100) has none.

So the overlay MTU is now 1280, which is also the floor. A smaller value is
refused when the plugin opens, naming the reason, rather than surfacing as
an obscure netlink error after the user has already run four commands.

That leaves no slack against the other constraint: a packet needs mtu + 32
bytes of transport datagram, so 1312. A direct QUIC path offers roughly
1380 and fits; a relayed path may not, so the plugin now reports the exact
numbers when a link cannot carry a full-size packet, instead of only
counting silent drops.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/bin/tsunagi.rs

@@ -83,7 +83,7 @@ struct TunSetupArgs {
     #[arg(long, default_value = "tsun")]
     wg_prefix: String,
 
-    /// Interface MTU, matching `tsunagi up --wg-mtu`.
+    /// Interface MTU, matching `tsunagi up --wg-mtu`. At least 1280.
     #[arg(long)]
     wg_mtu: Option<u32>,
 }
@@ -195,6 +195,8 @@ struct UpArgs {
     wg_prefix: String,
 
     /// Interface MTU for the WireGuard data plane.
+    ///
+    /// Must be at least 1280, the minimum IPv6 requires.
     #[arg(long)]
     wg_mtu: Option<u32>,