feat(server): make server alloc-free on bare_metal, and surface inbound non-SD unicast

Cargo.toml

@@ -109,7 +109,14 @@ _alloc = []
 # bringing `Arc<Mutex<E2ERegistry>>` / `Arc<RwLock<SubscriptionManager>>` /
 # / `TokioTransport` / `TokioTimer` defaults into scope, and forces
 # `std`.
-server = ["dep:futures-util", "_alloc"]
+#
+# `server` itself is alloc-free: the no-alloc path is
+# `Server::new_with_handles` + `run_with_buffers` with static handles and
+# a `&'static AtomicBool` run latch. The allocator-backed conveniences
+# (`new_with_deps`/`new_passive_with_deps`, `run`/`run_inner`'s 64 KiB
+# buffers, the `Arc` `StartedLatch`) are gated behind `_alloc`, which
+# `std` / `server-tokio` / `embassy_channels` still pull in.
+server = ["dep:futures-util"]
 server-tokio = ["server", "std", "dep:tokio", "dep:socket2"]
 # Marks a build as intended for bare-metal / no_std consumption.
 # Activates embassy-sync as the channel backend, the `static_channels`
@@ -169,6 +176,11 @@ harness = false
 name = "bare_metal_server"
 required-features = ["server", "bare_metal"]
 
+[[test]]
+name = "no_alloc_server_witness"
+required-features = ["server", "bare_metal"]
+harness = false
+
 [[test]]
 name = "bare_metal_e2e"
 required-features = ["client", "server", "bare_metal"]

examples/bare_metal_server/src/main.rs

@@ -260,6 +260,7 @@ async fn main() {
                 timer: MockTimer,
                 e2e_registry: e2e,
                 subscriptions: subs,
+                non_sd_observer: None,
             },
             config,
             false, // multicast_loopback

examples/embassy_net_client/src/main.rs

@@ -374,6 +374,7 @@ async fn main() {
                 timer: LocalTimer,
                 e2e_registry: server_e2e,
                 subscriptions: InMemorySubscriptions::default(),
+                non_sd_observer: None,
             };
 
             // Default `H = Arc<F::Socket>`. Annotation is explicit

src/lib.rs

@@ -225,7 +225,9 @@ pub use client::{
 // `ClientChannelTypes` elaboration limit at the wrong call site.
 pub use e2e::{E2ECheckStatus, E2EKey, E2EProfile};
 #[cfg(feature = "server")]
-pub use server::{Server, ServerDeps, ServerHandles, ServerStorage, SubscriptionHandle};
+pub use server::{
+    NonSdRequestCallback, Server, ServerDeps, ServerHandles, ServerStorage, SubscriptionHandle,
+};
 #[cfg(any(feature = "client-tokio", feature = "server-tokio"))]
 pub use tokio_transport::{TokioChannels, TokioSocket, TokioSpawner, TokioTimer, TokioTransport};
 #[cfg(feature = "bare_metal")]