Initial skeleton: ZMK module structure + Javelin submodule + stub shims

Phase 1 scaffold for porting Javelin steno engine as a ZMK module.
All platform shims are stubs — compiles but no steno processing yet.

include/zmk_javelin_steno/zmk_platform_shim.h

@@ -0,0 +1,12 @@
+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+void zmk_javelin_steno_init(void);
+void zmk_javelin_steno_process_key(int steno_key_index, bool is_press);
+
+#ifdef __cplusplus
+}
+#endif