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/cdchanger.hpp

@@ -6,6 +6,7 @@
 #pragma once
 
 #include <cstdint>
+#include <memory>
 
 #include "avclan.h"
 #include "device.hpp"
@@ -61,11 +62,11 @@ public:
   void init();
 
   void handle(const Frame *in, Frame *out);
-  void react(Frame *out, detail::Error::Send err);
+  std::unique_ptr<Frame>
+  react(expected<std::unique_ptr<Frame>, detail::SendError> exp);
   void enable(Frame *out);
   void disable(Frame *out);
   static bool pending();
-  static void resolvepending();
   void emit(Frame *out);
   void incrementTime();
   bool isPlaying() const;