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

.github/workflows/package-linux.yml

@@ -1,8 +1,13 @@
 name: Linux Package
 on:
   push:
+    branches:
+      - master
+      - maint
+      - next
     tags:
       - "v*"
+  pull_request:
   workflow_dispatch:
 
 env:
@@ -85,7 +90,7 @@ jobs:
         run: cargo install cargo-deb --version 3.6.3 --locked
 
       - name: Build release binaries
-        run: cargo build --release
+        run: cargo build --release --features gateway
 
       - name: Build systemd tarball
         env: