feat: Android-ready core: target_os gating and app-owned TUN seam

Make the FIPS core build and run as an embedded Android library. The host
app owns the TUN (e.g. an Android VpnService) and FIPS performs no
system-TUN or CAP_NET_ADMIN operations.

Squashed from the following changes:

- gate desktop transports/TUN by target_os, not features: a plain
  `cargo build` now compiles for every target with no flags. Ethernet (raw
  AF_PACKET / BPF) is gated to linux/macos, so Android (target_os =
  "android", not "linux") self-excludes it as Windows already did; real
  system-TUN ops are gated per linux/macos and Android gets a no-op stub;
  the ipi6_ifindex cast handles it being i32 on Android vs u32 on macOS. No
  Cargo features are introduced; desktop builds are unchanged.

- app-owned TUN seam: Node::enable_app_owned_tun() lets an embedder that
  owns the TUN fd exchange IPv6 packet bytes with FIPS over channels
  instead of FIPS creating a system TUN device. It returns (app_outbound_tx,
  app_inbound_rx): the embedder pushes packets read from its fd into the
  outbound sender (app -> mesh) and pulls packets destined for its fd from
  the inbound receiver (mesh -> app). start() gates system-TUN creation on
  tun_tx being unset, so with the channels pre-installed it skips device
  creation and does no system-TUN ops; both directions reuse the existing
  inbound-shim and run_rx_loop wiring. Packets entering via app_outbound_tx
  bypass handle_tun_packet, so the embedder must push only fd00::/8-destined
  packets and clamp TCP MSS on outbound SYNs; the rustdoc and the
  IPv6-adapter design doc spell this out.

- keep the android target warning-clean so the cross-compile check passes
  clippy -D warnings.

- add an Android cross-compile CI check: cross-compile the library for
  aarch64-linux-android via cargo-ndk and run clippy -D warnings. Android
  ships as an embedded library (the host app owns the TUN), so there is no
  daemon binary to package; this is a check job, not a packaging one.

- docs: list Android as a supported platform.

Tests: app_owned_tun_seam_wires_channels covers the channel round-trip and
the Active state; start_skips_system_tun_when_app_owned runs start() and
asserts no named system device is created.

README.md

@@ -112,17 +112,19 @@ tutorial progression starting at
 cargo build --release
 ```
 
-Requires Rust 1.94.1+ (edition 2024). Linux, macOS, and Windows are
-supported; transport availability varies by platform.
+Requires Rust 1.94.1+ (edition 2024). Linux, macOS, and Windows run as
+standalone daemons; Android is supported as an embedded library (the host
+app owns the TUN, e.g. a `VpnService`). Transport availability varies by
+platform.
 
-| Transport | Linux | macOS | Windows | OpenWrt |
-|-----------|:-----:|:-----:|:-------:|:-------:|
-| UDP       |   ✅  |   ✅  |    ✅   |   ✅    |
-| TCP       |   ✅  |   ✅  |    ✅   |   ✅    |
-| Ethernet  |   ✅  |   ✅  |    ❌   |   ✅    |
-| Tor       |   ✅  |   ✅  |    ✅   |   ✅    |
-| Nym       |   ✅  |   ✅  |    ✅   |   ❌    |
-| BLE       |   ✅  |   ❌  |    ❌   |   ❌    |
+| Transport | Linux | macOS | Windows | Android | OpenWrt |
+|-----------|:-----:|:-----:|:-------:|:-------:|:-------:|
+| UDP       |   ✅  |   ✅  |    ✅   |    ✅   |   ✅    |
+| TCP       |   ✅  |   ✅  |    ✅   |    ✅   |   ✅    |
+| Ethernet  |   ✅  |   ✅  |    ❌   |    ❌   |   ✅    |
+| Tor       |   ✅  |   ✅  |    ✅   |    ❌   |   ✅    |
+| Nym       |   ✅  |   ✅  |    ✅   |    ❌   |   ❌    |
+| BLE       |   ✅  |   ❌  |    ❌   |    ❌   |   ❌    |
 
 On Linux, a source build requires `libclang` — the LAN gateway's
 nftables bindings are generated by `bindgen` at build time, which