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>

cmake/apply-patch.cmake

@@ -0,0 +1,18 @@
+# Idempotent patch driver for FetchContent PATCH_COMMAND steps.
+#
+# Usage: cmake -DGIT_EXECUTABLE=<git> -DPATCH_FILE=<patch> -P apply-patch.cmake
+# (run with CWD = the populated source directory, as PATCH_COMMAND does).
+#
+# FetchContent re-runs PATCH_COMMAND after its update step on reconfigure, so a
+# bare `git apply` fails the second time around. Skip if the patch is already
+# applied (the reverse-apply dry run succeeds); otherwise apply it for real.
+execute_process(
+    COMMAND "${GIT_EXECUTABLE}" apply --reverse --check --ignore-whitespace
+        "${PATCH_FILE}"
+    RESULT_VARIABLE _already_applied
+    OUTPUT_QUIET ERROR_QUIET)
+if(NOT _already_applied EQUAL 0)
+    execute_process(
+        COMMAND "${GIT_EXECUTABLE}" apply --ignore-whitespace "${PATCH_FILE}"
+        COMMAND_ERROR_IS_FATAL ANY)
+endif()

cmake/tl-expected-no-exception-header.patch

@@ -0,0 +1,36 @@
+diff --git a/include/tl/expected.hpp b/include/tl/expected.hpp
+index 59e59aa..75857bd 100644
+--- a/include/tl/expected.hpp
++++ b/include/tl/expected.hpp
+@@ -20,7 +20,7 @@
+ #define TL_EXPECTED_VERSION_MINOR 3
+ #define TL_EXPECTED_VERSION_PATCH 1
+ 
+-#include <exception>
++#include <cstdlib>
+ #include <functional>
+ #include <type_traits>
+ #include <utility>
+@@ -222,7 +222,7 @@ static constexpr unexpect_t unexpect{};
+ #ifdef TL_EXPECTED_EXCEPTIONS_ENABLED
+ #define TL_EXPECTED_THROW_EXCEPTION(e) throw((e));
+ #else
+-#define TL_EXPECTED_THROW_EXCEPTION(e) std::terminate();
++#define TL_EXPECTED_THROW_EXCEPTION(e) std::abort();
+ #endif
+ 
+ namespace detail {
+@@ -1249,11 +1249,11 @@ template <class T, class E> struct expected_default_ctor_base<T, E, false> {
+ };
+ } // namespace detail
+ 
+-template <class E> class bad_expected_access : public std::exception {
++template <class E> class bad_expected_access {
+ public:
+   explicit bad_expected_access(E e) : m_val(std::move(e)) {}
+ 
+-  virtual const char *what() const noexcept override {
++  virtual const char *what() const noexcept {
+     return "Bad expected access";
+   }
+ 

cmake/tl-optional-no-exception-header.patch

@@ -0,0 +1,21 @@
+diff --git a/include/tl/optional.hpp b/include/tl/optional.hpp
+index e9c59c2..1ff3d4a 100644
+--- a/include/tl/optional.hpp
++++ b/include/tl/optional.hpp
+@@ -21,7 +21,6 @@
+ #define TL_OPTIONAL_VERSION_MINOR 1
+ #define TL_OPTIONAL_VERSION_PATCH 0
+ 
+-#include <exception>
+ #include <functional>
+ #include <new>
+ #include <type_traits>
+@@ -664,7 +663,7 @@ struct nullopt_t {
+ static constexpr nullopt_t nullopt{nullopt_t::do_not_use{},
+                                    nullopt_t::do_not_use{}};
+ 
+-class bad_optional_access : public std::exception {
++class bad_optional_access {
+ public:
+   bad_optional_access() = default;
+   const char *what() const noexcept { return "Optional has no value"; }