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/hostrules/mod.rs

@@ -64,8 +64,11 @@ mod windows;
 #[cfg(all(feature = "tun-device", target_os = "windows"))]
 pub use windows::WindowsHostRules;
 
+// Compiled everywhere so its rule text and parsers are tested everywhere.
 #[cfg(all(feature = "tun-device", target_os = "macos"))]
 mod macos;
+#[cfg_attr(not(target_os = "macos"), allow(dead_code))]
+mod macos_exit;
 #[cfg(all(feature = "tun-device", target_os = "macos"))]
 pub use macos::MacosHostRules;