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/bus.hpp

@@ -48,9 +48,11 @@
 #pragma once
 
 #include <cstdint>
+#include <memory>
 
 #include "avclan.h"
 #include "frame.hpp"
+#include "stdshim.hpp"
 
 namespace avclan {
 
@@ -77,7 +79,8 @@ public:
   void measure();
 #endif
 
-  Error::Read read(uint16_t address, Frame *in, Frame::Print print);
+  expected<std::unique_ptr<Frame>, Error::Read> read(uint16_t address,
+                                                     Frame::Print print);
   Error::Send send(const Frame *out, Frame::Print print);
 
 private: