README: note the c-abi host-strip change shipped in 2026.9.18.3

The 2026.9.18.3 release (PR #673) is the floor for any package that
declares `[c-abi]` --- `openkal-musl` 0.15.0 today, anything else
that comes after. The Windows host's clang would inject
`_WIN32` / `_WIN64` / `__MINGW32__` / `__MINGW64__` into the
preprocessor output even with `--target=` set, and the host's
default wchar width is not trustworthy on freestanding. The fix
(`cenv_probe::verify` hostStripMacros, freestanding wchar always
emits `-fno-short-wchar`) is engine-side; older engines silently
misbuild `[c-abi]` packages without it. Pin the upgrade note so
a reader landing on the README from a search knows why the bump is
required.

Co-Authored-By: Claude Code <noreply@anthropic.com>

Author: speak-agent

README.md

@@ -18,6 +18,14 @@
   <img src="https://github.com/user-attachments/assets/6c85896e-9a37-4f62-acfb-d37a4eae2363" alt="mcpp demo" width="720">
 </p>
 
+> **Note (2026.9.18.3):** this release adds `cenv_probe::verify` host-macro
+> stripping for the Windows host and forces `-fno-short-wchar` on freestanding
+> wchar realisation. `[c-abi]` packages (`openkal-musl` 0.15.0 and any future
+> C-library package) require this engine. Older engines silently misbuild them:
+> the c-abi probe sees host contamination (`_WIN32`, `__MINGW32__`,
+> `__MINGW64__`, `_WIN64`) on Windows and reads the wrong `__SIZEOF_WCHAR_T__`
+> on a freestanding target. Upgrade: `xlings install mcpp --force`.
+
 ## Highlights
 
 - **Modular build system** — C++ modules first: `import std` handled automatically, file-level incremental builds, automatic dependency analysis, nothing to configure