chore: add TDD workflow, behavioral guidelines, and ASan support

Add deps/Sanitizers.cmake for AddressSanitizer support (ENABLE_ASAN).
Add TDD workflow and behavioral guidelines to AGENTS.md.

deps/Sanitizers.cmake

@@ -0,0 +1,28 @@
+# ==============================================================================
+# Sanitizers
+# ==============================================================================
+# AddressSanitizer (ASan) support.
+# Works with GCC/Clang on Linux and macOS, Clang on Windows (requires
+# compiler-rt with sanitizers), and MSVC on Windows (/fsanitize=address).
+#
+# Usage: cmake -DENABLE_ASAN=ON ...
+# ==============================================================================
+
+option(ENABLE_ASAN "Build with AddressSanitizer" OFF)
+
+if (ENABLE_ASAN)
+    if (ENABLE_COVERAGE)
+        message(FATAL_ERROR "ENABLE_ASAN and ENABLE_COVERAGE cannot be used together")
+    endif()
+
+    if (MSVC)
+        add_compile_options(/fsanitize=address)
+        message(STATUS "ASan: enabled (MSVC)")
+    elseif (CMAKE_C_COMPILER_ID MATCHES "GNU|Clang")
+        add_compile_options(-fsanitize=address -fno-omit-frame-pointer)
+        add_link_options(-fsanitize=address)
+        message(STATUS "ASan: enabled (${CMAKE_C_COMPILER_ID})")
+    else()
+        message(FATAL_ERROR "ENABLE_ASAN: unsupported compiler ${CMAKE_C_COMPILER_ID}")
+    endif()
+endif()