Exit nodes on macOS: pf anchor for offering, routes plus route-to for using

The exit-node host rules existed only for Linux (iptables + policy routing);
macOS reported "only supported on Linux yet".

Offering: a pf anchor com.apple/tsunagi-exit-<if> (evaluated by the stock
pf.conf) holds a nat rule per offered range and a pass-in rule. pf is enabled
if needed and the pfctl -E reference is released when the rules are removed.
net.inet.ip.forwarding is only read, never written, as on Linux.

Using: macOS has no uidrange policy routing and the agent is root, so the
overlay gets 0.0.0.0/1 and 128.0.0.0/1 routes and a pf route-to rule (matched
by the agent's uid) sends the agent's own QUIC/relay traffic back out the
physical interface, translated to its address. The rules are loaded before the
routes, so a failed load never sends traffic into a tunnel it cannot exempt
the agent from. IPv6 is blocked with rejecting ::/1 and 8000::/1 routes.
ExitHostPlan gains `overlay` (every network range) for the route-to exemption;
Linux ignores it.

Known limits: the egress is the default route's interface at apply time; other
root processes also bypass the tunnel. Forwarding hints in the CLI and tray
name the macOS sysctl. The rule text and command order are unit-tested with a
scripted host; nothing was run against a live pf/route (needs root), so the
route-to + nat second-pass behaviour is unverified.

Checks: cargo fmt --check, cargo clippy --locked --workspace --all-targets
-D warnings and cargo test --locked --workspace --all-targets, run on macOS
only.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/tsunagi-core/src/overlay/interface.rs

@@ -354,6 +354,7 @@ impl Interface {
             interface: self.name.clone(),
             offer: ranges,
             client: self.routes.exit_via().is_some(),
+            overlay: self.routes.overlay_ranges(),
         };
 
         let first = !std::mem::replace(&mut self.exit_state().cleaned, true);
@@ -398,9 +399,13 @@ impl Interface {
             tracing::warn!(
                 interface = %self.name,
                 "kernel forwarding is off for this interface, so nothing is forwarded for the \
-                 exit node yet: `sysctl -w net.ipv4.conf.{0}.forwarding=1` (or \
-                 `net.ipv4.ip_forward=1`), and set it in sysctl.d to keep it",
-                self.name
+                 exit node yet: {}",
+                if cfg!(target_os = "macos") {
+                    "`sysctl -w net.inet.ip.forwarding=1`, set again after every reboot"
+                } else {
+                    "`sysctl -w net.ipv4.ip_forward=1` (or `net.ipv4.conf.<interface>.forwarding=1`), \
+                     and set it in sysctl.d to keep it"
+                }
             );
         }
         match &report.client {
@@ -443,7 +448,7 @@ impl Interface {
                 "this overlay interface is not on the host (no TUN device), so there is \
                  nothing to forward to",
             ),
-            Some(false) => Some("exit nodes are only supported on Linux yet"),
+            Some(false) => Some("exit nodes are not supported on this platform yet"),
             Some(true) => None,
         };
         let failed = |detail: &str| RuleSetReport {