Add macOS support, fix bloom filter routing and MMP intervals

macOS platform:
- Platform-native TUN interface management with shutdown pipe
- Raw Ethernet transport with macOS socket backend (socket_macos.rs)
- EthernetTransport and TransportHandle::Ethernet ungated from Linux-only
- macOS .pkg packaging (build-pkg.sh, launchd plist, uninstall script)
- CI: macOS build and unit test jobs; x86_64 cross-compiled from
  macos-latest via rustup target add x86_64-apple-darwin

Gateway feature flag:
- New opt-in `gateway` Cargo feature activates optional `rustables` dep
- `pub mod gateway` and `Config.gateway` gated behind the feature so
  macOS builds never pull in Linux-only nftables bindings
- `fips-gateway` bin has `required-features = ["gateway"]`
- All Linux/OpenWrt/AUR packaging passes `--features gateway`

CI / packaging:
- package-linux, package-macos, package-openwrt now trigger on push to
  master/maint/next and on pull requests; release uploads remain tag-gated
- Bloom filter routing fix: fall through to tree routing when no candidate
  is strictly closer
- MMP intervals: raise MIN to 1s / MAX to 5s with 5-sample cold-start phase

packaging/debian/build-deb.sh

@@ -71,7 +71,7 @@ echo "Building .deb package..."
 OUTPUT_DIR="$(mktemp -d)"
 trap 'rm -rf "${OUTPUT_DIR}"' EXIT
 
-cargo_args=(deb --output "${OUTPUT_DIR}")
+cargo_args=(deb --output "${OUTPUT_DIR}" --features gateway)
 if [[ -n "${TARGET_TRIPLE}" ]]; then
     cargo_args+=(--target "${TARGET_TRIPLE}")
 fi