Reduce payload size limit from 240 → 64 bytes (#2)

* Initial plan

* Reduce payload size limit from 240 to 64 bytes (closes latent _rawIdx overflow)

Co-authored-by: petesramek <2333452+petesramek@users.noreply.github.com>
Agent-Logs-Url: https://github.com/petesramek/tiny-link/sessions/a7a7c918-f73f-48b1-bdf7-b7dcd87fe2ab

* Fix AVR build: guard SFINAE adapter validation behind `#ifndef __AVR__` (#4)

* Initial plan

* Guard SFINAE block with #ifndef __AVR__ to fix ATtiny88 CI build

Co-authored-by: petesramek <2333452+petesramek@users.noreply.github.com>
Agent-Logs-Url: https://github.com/petesramek/tiny-link/sessions/c5178801-5f58-4aba-afeb-0100e94e1cbf

* Updated SFINAE validation in TinyLink.h to use std::declval<T>() instead of T() for improved adapter validation.

* Updated test/test_protocol.cpp with changes to tinylink::TYPE_DATA

* Append missing main() function

* Revert "Append missing main() function"

This reverts commit bb18bb0.

* fixed tesst

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: petesramek <2333452+petesramek@users.noreply.github.com>
Co-authored-by: Pete Sramek <me@petesramek.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: petesramek <2333452+petesramek@users.noreply.github.com>
Co-authored-by: Pete Sramek <me@petesramek.com>

Author: Copilot

CHANGELOG.md

@@ -25,6 +25,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 - `CONTRIBUTING.md`, `CODE_OF_CONDUCT.md`, `SECURITY.md`, `CHANGELOG.md`, issue/PR templates, and `CITATION.cff` for OSS best practices.
 
+### Changed
+- Payload size limit reduced from 240 bytes to 64 bytes to enforce micro-message design intent and eliminate a latent `_rawIdx` overflow bug on large payloads.
+
 ---
 
 ## [0.4.0] – 2026-01-01

src/TinyLink.h

@@ -1,4 +1,4 @@
-/**
+/**
  * @file TinyLink.h
  * @brief Main template engine for the TinyLink protocol.
  */
@@ -11,34 +11,32 @@
 #include <stdint.h>
 #include <string.h>
 
-namespace tinylink {
+#ifndef __AVR__
+#include <type_traits>
+#endif
 
+namespace tinylink {
 
+#ifndef __AVR__
     // -----------------------------------------------------------------------------
-    //  SFINAE Adapter Validation (C++11/14 compatible)
+    //  SFINAE Adapter Validation (C++11/14 compatible, non-AVR only)
     // -----------------------------------------------------------------------------
     //
-    // This ensures the Adapter type implements:
-    //   - bool isOpen()
-    //   - int  available()
-    //   - int  read()
-    //   - void write(uint8_t)
-    //   - void write(const uint8_t*, size_t)
-    //   - unsigned long millis()
-    // WITHOUT dereferencing a null pointer or requiring C++20 concepts.
+    // avr-gcc does not ship <type_traits>, so this block is skipped on AVR targets.
+    // On desktop/ESP builds, this enforces the adapter contract at compile time.
+    // std::declval<T>() is used so the adapter need not be default-constructible.
     //
-
     template <typename A>
     struct is_valid_adapter {
     private:
         template <typename T>
         static auto check(int) -> decltype(
-            (void) static_cast<bool>(T().isOpen()),
-            (void) static_cast<int>(T().available()),
-            (void) static_cast<int>(T().read()),
-            (void)T().write(uint8_t(0)),
-            (void)T().write((const uint8_t*)0, size_t(0)),
-            (void) static_cast<unsigned long>(T().millis()),
+            (void) static_cast<bool>(std::declval<T>().isOpen()),
+            (void) static_cast<int>(std::declval<T>().available()),
+            (void) static_cast<int>(std::declval<T>().read()),
+            (void)std::declval<T>().write(uint8_t(0)),
+            (void)std::declval<T>().write((const uint8_t*)0, size_t(0)),
+            (void) static_cast<unsigned long>(std::declval<T>().millis()),
             std::true_type{}
             );
 
@@ -52,6 +50,18 @@ namespace tinylink {
     template <typename A>
     using adapter_check_t = typename std::enable_if<is_valid_adapter<A>::value, int>::type;
 
+#else
+    // -----------------------------------------------------------------------------
+    //  AVR: No SFINAE validation (avr-gcc has no <type_traits>)
+    // -----------------------------------------------------------------------------
+    //
+    // On AVR targets the adapter contract is trusted to be correct by the user.
+    // The template parameter is preserved for API compatibility.
+    //
+    template <typename A>
+    using adapter_check_t = int;
+
+#endif
 
     // -----------------------------------------------------------------------------
     //  TinyLink Engine Template
@@ -163,14 +173,13 @@ namespace tinylink {
         unsigned long _lastByte = 0;
         unsigned long _timeout = 250;
 
-
     public:
 
         // -------------------------------------------------------------------------
         // Constructor
         // -------------------------------------------------------------------------
         TinyLink(Adapter& hw) : _hw(&hw) {
-            static_assert(sizeof(T) <= 240, "TinyLink: Payload exceeds 240 bytes (COBS limit).");
+            static_assert(sizeof(T) <= 64, "TinyLink: Payload exceeds 64 bytes. TinyLink is designed for micro-messages.");
             static_assert(alignof(T) == 1, "TinyLink: Data type must be packed. Use __attribute__((packed)).");
         }