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/stdshim.hpp

@@ -0,0 +1,25 @@
+// Copyright (C) 2026 Allen Hill <allenofthehills@gmail.com>
+// SPDX-License-Identifier: GPL-3.0-or-later
+#pragma once
+
+#include <version>
+
+#if !defined(__cpp_lib_expected) || __cpp_lib_expected < 202211L
+  #include <tl/expected.hpp>
+
+namespace avclan {
+template <class T, class E> using expected = tl::expected<T, E>;
+template <class E> using unexpected = tl::unexpected<E>;
+using tl::unexpect;
+using tl::unexpect_t;
+} // namespace avclan
+#else
+  #include <expected>
+
+namespace avclan {
+template <class T, class E> using expected = std::expected<T, E>;
+template <class E> using unexpected = std::unexpected<E>;
+using std::unexpect;
+using std::unexpect_t;
+} // namespace avclan
+#endif