Phase19e loopback integration test

[JustinKovacich]

Validates 19a-c against a real embassy_net::Stack:

• LoopbackDriver pair: two in-memory Pipes (queue + waker)
  bridge two embassy_net::Stack instances. No kernel TUN, no
  privileges; runs in any CI without setup. HardwareAddress::Ip
  medium skips ARP/Ethernet — pure IP traffic over the loopback.
• adapter_udp_roundtrip: two stacks on 169.254.1.1 / .1.2,
  two EmbassyNetFactory + SocketPool pairs, bind a socket on
  each, send a UDP datagram A→B, assert byte-equality + source
  address. Tightest end-to-end exercise of bind / send_to /
  recv_from / local_addr / SocketPool slot lifecycle.
• Runtime: #[tokio::test(flavor = "current_thread")] + a
  LocalSet driving per-stack spawn_local runners.
  Stack<LoopbackDriver> is !Sync (RefCell internals), so
  Stack::run() is !Send — multi-thread tokio::spawn does
  not type-check. The current_thread flavor matches the
  single-task model bare-metal targets actually run under.
• Cargo: adds tokio (rt-multi-thread, macros, time, sync) and
  futures to dev-deps.

What this leaves for follow-on phases:

• 19f — Server::new_with_deps_local + parallel impl Server
  block with relaxed Send + Sync bounds. Required because
  embassy_net::udp::UdpSocket<'static> is !Sync (borrows
  from Stack's RefCell<Inner<D>>), and Server's existing
  three impl blocks (mod.rs:275/430/1065) all require
  F::Socket: Send + Sync. Mirrors Client's existing
  new_with_deps_local pattern.
• 19g — SOME/IP Client+Server integration test. Lifts the
  tests/bare_metal_e2e.rs harness onto the loopback stack
  pair using the 19f _local API. Mirrors the parent crate's
  client_receives_server_sd_announcement and
  client_send_request_server_runloop_stable, with
  EmbassyNetFactory swapped in for MockFactory.

Scope split rationale: per phase_13_5_lessons.md lesson #2,
"abstract over X" and "drop X" are separate commitments —
bundling the Server bound-relaxation under "loopback test"
would repeat the v2 phases-11/13a aspirational-gate mistake.

Plan v3 updated 2026-04-29 (memory) with the 19e/f/g/h/i/j
re-numbering.

Gates green:

• cargo fmt --check
• cargo clippy -p simple-someip-embassy-net --all-targets -D warnings
• cargo test -p simple-someip-embassy-net --test loopback
• cargo build -p simple-someip-embassy-net --target thumbv7em-none-eabihf
• cargo check --workspace --all-targets

[Copilot]

Pull request overview

Adds a host-runnable loopback integration test to validate the simple-someip-embassy-net transport adapter against a real embassy_net::Stack, without requiring a kernel TUN/TAP device or special privileges.

Changes:

• Introduces tests/loopback.rs with an in-memory LoopbackDriver bridging two embassy_net::Stack instances and a UDP roundtrip assertion.
• Updates simple-someip-embassy-net dev-dependencies to support the Tokio-based test harness.
• Refreshes Cargo.lock for newly added dev-deps.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 4 comments.

File	Description
simple-someip-embassy-net/tests/loopback.rs	Implements a software loopback Driver and a Tokio LocalSet-driven UDP roundtrip test across two stacks.
simple-someip-embassy-net/Cargo.toml	Updates host-test dev-deps and adds Tokio (+ futures) for the new test harness.
Cargo.lock	Locks new transitive dependencies introduced by the dev-dependency updates.

---

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[Copilot]

These comments say the harness uses tokio::spawn for the per-stack Stack::run() tasks, but the test uses tokio::task::spawn_local (required by LocalSet / current_thread). Updating the comment will avoid confusion about why rt-multi-thread isn’t actually needed for the test.

Suggested change

	# `tokio::spawn` for the per-stack `Stack::run()` futures, and
	# `tokio::time::timeout` for bounded assertions. Same shape as the
	# parent crate's `tests/bare_metal_e2e.rs` harness.
	# `tokio::task::spawn_local` for the per-stack `Stack::run()` futures
	# within a `LocalSet` / current-thread runtime, and
	# `tokio::time::timeout` for bounded assertions. Same overall shape as
	# the parent crate's `tests/bare_metal_e2e.rs` harness.

[Copilot]

The crate adds futures = "0.3" as a dev-dependency, but there’s no code in simple-someip-embassy-net (including tests/loopback.rs) that actually uses the futures crate right now. Consider dropping this dev-dep until it’s needed (or add the missing usage) to keep the dependency surface minimal.

Suggested change

	# `futures` brings `select_biased!` / `FusedFuture` / `pin_mut!` into
	# scope for the test driver.
	futures = "0.3"

[Copilot]

The module docs claim this test exercises local_addr end-to-end, but the test body never calls local_addr(). Either add an assertion that checks sock_a.local_addr() / sock_b.local_addr() matches the bound address, or adjust the docs to reflect what’s actually covered.

Suggested change

	//! `send_to` / `recv_from` / `local_addr` end-to-end.
	//! `send_to` / `recv_from` end-to-end.

[Copilot]

This doc comment says build_stack returns a separately-leaked future to tokio::spawn, but the function only returns &'static Stack<LoopbackDriver>. Update the comment (or function signature) so the documentation matches the actual API.

Suggested change

	/// Build a stack on `ip/24` with our `LoopbackDriver`. Returns a
	/// `&'static Stack<LoopbackDriver>` ready for `EmbassyNetFactory`
	/// and a separately-leaked future to `tokio::spawn` for the
	/// stack's run loop.
	/// Build a stack on `ip/24` with our `LoopbackDriver` and return a
	/// leaked `&'static Stack<LoopbackDriver>` ready for
	/// `EmbassyNetFactory`.