Redesign (simplify) top-level avclan interface to use expected/nullable unique_ptr

- Peripheral::read/send now return `expected<unique_ptr<Frame>, Error>`
for a unified success/error interface.
- Peripheral::route returns `expected<unique_ptr<Frame>, Error>` to
distinguish intent: (intentional) non-response vs unable to respond
- Other functions with optional message semantics (handle,poll,react)
return a unique_ptr whose ownership-state indicates response intent
(i.e. send new message)

Bundled necessary changes:
- Switch Frame allocation from global to local (enabled via member
  function new/delete)
- Add new header-library tl::expected to shim avr-libstdcpp

Unrelated: Defensive `continue` added after `route`, to reduce Bus
activity check latency

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/avclan/device.hpp

@@ -5,9 +5,11 @@
 
 #include <concepts>
 #include <cstdint>
+#include <memory>
 
 #include "avclan.h"
 #include "frame.hpp"
+#include "stdshim.hpp"
 
 namespace avclan {
 
@@ -46,15 +48,19 @@ enum class Device : uint8_t {
 };
 
 template <class T>
-concept DeviceInterface = requires {
-  std::integral_constant<Device, T::id>{};
-} && requires(T dev, const Frame *in, Frame *out, detail::Error::Send err) {
-  dev.init();
-  dev.handle(in, out);
-  dev.enable(out);
-  dev.react(out, err);
-  { dev.pending() } -> std::convertible_to<bool>;
-  dev.resolvepending();
-  dev.emit(out);
-};
+concept DeviceInterface =
+    requires { std::integral_constant<Device, T::id>{}; } &&
+    requires(T dev, const Frame *in, Frame *out,
+             expected<std::unique_ptr<Frame>, detail::SendError> exp) {
+      dev.init();
+      dev.handle(in, out);
+      dev.enable(out);
+      {
+        dev.react(std::move(exp))
+      } -> std::same_as<std::unique_ptr<Frame>>;
+
+      { dev.pending() } -> std::convertible_to<bool>;
+      // Devices must clear `pending()` after `emit()` is called
+      dev.emit(out);
+    };
 } // namespace avclan