Module runtime 

Async runtime abstraction (Phase 3.1).

phantom_protocol historically hard-coded tokio everywhere: every tokio::spawn, every tokio::time::sleep, every tokio::sync::Mutex, every tokio::net::TcpStream. That couples the library to a multi-threaded executor that does not exist on wasm32-unknown-unknown (single-threaded, JS event loop) or on bare-metal embedded targets (no executor at all without embassy/RTIC).

This module introduces a small trait surface — Runtime — that the rest of the crate can use in place of direct tokio calls. The default implementation, TokioRuntime, preserves today’s behavior verbatim. Follow-up commits will gradually migrate call sites; this commit lands only the trait + the default impl + tests.

What the trait covers

• Task spawning — spawn(boxed_future) -> SpawnHandle. The handle exposes a non-blocking abort() so callers can cancel the task.
• Sleep — sleep(duration) -> BoxFuture<()> for delay loops.
• Monotonic clock — now_monotonic() -> Instant for RTT and expiry math.
• Wall-clock — now_wall_clock() -> SystemTime for cookie buckets and timestamp-bound material.

What the trait deliberately does NOT cover (yet)

• Channels — tokio::sync::mpsc is portable enough across tokio and tokio derivatives that we keep using it directly.
• Mutexes — see above.
• Network I/O — this is the crate::transport::legs trait’s job. TcpStream / UdpSocket are leg-impl details, not runtime-level.

Implementations

Impl	Status	Target
TokioRuntime	✅	Linux / macOS / Windows / iOS / Android servers and clients
WasmRuntime	✅ (cfg(target_arch = "wasm32"))	wasm32-unknown-unknown browsers via wasm-bindgen-futures
EmbeddedRuntime	scaffold	embassy / bare metal

The non-tokio implementations are scaffolded under future feature flags; this commit only ships the trait and the tokio default so the call-site migration can begin against a stable abstraction.

Structs

SpawnHandle

Opaque handle to a spawned task. Created by Runtime::spawn.

TokioRuntime

Tokio-backed Runtime. Zero-sized — construct with TokioRuntime and wrap in Arc to share across tasks.

Traits

Runtime

Async runtime abstraction.

SpawnHandleInner

Implementation detail of SpawnHandle. Runtime adapters implement this on their concrete handle type.

Type Aliases

BoxFuture

Boxed, owned, Send future of unit output — the shape Runtime::spawn and Runtime::sleep work in.