docs: add C++ scratchpad notes

Collect new C++ features to cover, design patterns, and compiler
options for catching undefined behavior.

Co-authored-by: Qwen 3.8 27B <qwen3.8_27b-q3_k_xl@lmstudio.local>

notes/scratch.md

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+# C++ Scratchpad
+
+## New features in C++
+
+- 23 and 26
+- r-value (and the values that I forgor)
+
+## Patterns
+
+- CRTP
+
+## compiler options that make undefined an error
+
+- -fsanitize=undefined
+- chk_add(), chkd_sub(), chkd_mul()
+- non-portable ways to check CPU overflow
+  - msvc intsafe.h
+
+## Design Patterns