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>.

gmake.mk

@@ -61,17 +61,20 @@ test: debug
 # Installation (Unix only)
 ifneq ($(OS),Windows_NT)
 install: release
-	install -d $(LIBDIR) $(INCDIR)
+	install -d $(LIBDIR) $(INCDIR)/stk
 	install -m 755 $(BIN_DIR)/release/$(FULL_LIB) $(LIBDIR)/
-	install -m 644 $(INC_DIR)/stk.h $(INCDIR)/
+	install -m 644 $(INC_DIR)/stk.h $(INCDIR)/stk/
+	install -m 644 $(INC_DIR)/stk_version.h $(INCDIR)/stk/
+	install -m 644 $(INC_DIR)/stk_log.h $(INCDIR)/stk/
 
 uninstall:
 	rm -f $(LIBDIR)/$(FULL_LIB)
-	rm -f $(INCDIR)/stk.h
+	rm -rf $(INCDIR)/stk
 else
 install:
 	@echo "make install is not supported on Windows."
-	@echo "Copy include/stk.h and bin/release/stk.dll to your project."
+	@echo "Copy include/ directory contents to your_project/include/stk/"
+	@echo "Copy bin/release/stk.dll to your project's lib directory."
 
 uninstall:
 	@echo "make uninstall is not supported on Windows."