rt: move slice append helpers from lib/slices/ into libwwrt.a

Makefile

@@ -39,7 +39,8 @@ W6L_SRC	= cmd/w6l/main.c cmd/w6l/obj.c cmd/w6l/sym.c cmd/w6l/pass.c cmd/w6l/out.
 W6L_OBJ	= $(W6L_SRC:cmd/w6l/%.c=$(OBJ)/w6l/%.o)
 
 RT_S	= rt/start.s rt/syscall.s rt/alloc.s rt/streq.s rt/abort.s
-RT_OBJ	= $(RT_S:rt/%.s=$(OBJ)/rt/%.o)
+RT_WW	= rt/append.ww
+RT_OBJ	= $(RT_S:rt/%.s=$(OBJ)/rt/%.o) $(RT_WW:rt/%.ww=$(OBJ)/rt/%.o)
 
 # Cstage: one-time C bootstrap (see BOOTSTRAP.md). Built by `cc`.
 # Goes away at v1.0, replaced by a checked-in stage-0 binary.
@@ -191,6 +192,14 @@ $(BIN)/ww_ww: selfhost/cmd/ww/main.ww selfhost/cmd/wcc/mem.ww lib/os/os.ww \
 $(OBJ)/rt/%.o: rt/%.s $(BIN)/w6a | $(OBJ)/rt
 	$(BIN)/w6a -o $@ $<
 
+# ww-side runtime helpers (rt/*.ww): compile via w6c to .s, then w6a.
+# Each .ww module is standalone — uses @symbol FFI for rt_alloc/rt_free
+# rather than `use os;` so no bundling is required.
+$(OBJ)/rt/%.s: rt/%.ww $(BIN)/w6c | $(OBJ)/rt
+	$(BIN)/w6c $< > $@
+$(OBJ)/rt/%.o: $(OBJ)/rt/%.s $(BIN)/w6a | $(OBJ)/rt
+	$(BIN)/w6a -o $@ $<
+
 $(LIB)/libwwrt.a: $(RT_OBJ) | $(LIB)
 	$(AR) rcs $@ $(RT_OBJ)