fix(install): install headers to stk/ subdirectory for proper namespacing

- Install headers to /include/stk/ instead of /include/
- Update README examples to use #include <stk/stk.h>
- Update Windows install instructions to reflect stk/ directory structure
- Add MPL-2.0 license badge to README
- Clarify DLL can be placed in binary or lib directory on Windows

This prevents header name collisions with other libraries and follows
standard library distribution conventions (similar to SDL).
Users now include stk headers as <stk/stk.h> rather than <stk.h>.

bmake.mk

@@ -58,10 +58,12 @@ test: debug
 	cd ${.CURDIR}/test && ${MAKE} -f bmake.mk
 
 install: release
-	install -d ${LIBDIR} ${INCDIR}
+	install -d ${LIBDIR} ${INCDIR}/stk
 	install -m 755 ${.CURDIR}/${BIN_DIR}/release/${FULL_LIB} ${LIBDIR}/
-	install -m 644 ${.CURDIR}/${INC_DIR}/stk.h ${INCDIR}/
+	install -m 644 ${.CURDIR}/${INC_DIR}/stk.h ${INCDIR}/stk/
+	install -m 644 ${.CURDIR}/${INC_DIR}/stk_version.h ${INCDIR}/stk/
+	install -m 644 ${.CURDIR}/${INC_DIR}/stk_log.h ${INCDIR}/stk/
 
 uninstall:
 	rm -f ${LIBDIR}/${FULL_LIB}
-	rm -f ${INCDIR}/stk.h
+	rm -rf ${INCDIR}/stk