fix(qbe): any-child terminator check in QBE_IsTerminator for hBlock — QBE SSA now fully valid

- QBE_IsTerminator(hBlock) checks ALL children, not just last
- Removed legacy c_backend.bux and nim_backend.bux from _selfhost/src/
- QBE self-hosting QBE phase: PASSES (SSA valid, assembled to .s)
- Remaining: linker errors from missing String_Eq/StringBuilder in runtime

vendor/qbe/Makefile

@@ -0,0 +1,93 @@
+.POSIX:
+.SUFFIXES: .o .c
+
+PREFIX = /usr/local
+BINDIR = $(PREFIX)/bin
+
+COMMOBJ  = main.o util.o parse.o abi.o cfg.o mem.o ssa.o alias.o load.o \
+           copy.o fold.o simpl.o live.o spill.o rega.o emit.o
+AMD64OBJ = amd64/targ.o amd64/sysv.o amd64/isel.o amd64/emit.o
+ARM64OBJ = arm64/targ.o arm64/abi.o arm64/isel.o arm64/emit.o
+RV64OBJ  = rv64/targ.o rv64/abi.o rv64/isel.o rv64/emit.o
+OBJ      = $(COMMOBJ) $(AMD64OBJ) $(ARM64OBJ) $(RV64OBJ)
+
+SRCALL   = $(OBJ:.o=.c)
+
+CC       = cc
+CFLAGS   = -std=c99 -g -Wall -Wextra -Wpedantic
+
+qbe: $(OBJ)
+	$(CC) $(LDFLAGS) $(OBJ) -o $@
+
+.c.o:
+	$(CC) $(CFLAGS) -c $< -o $@
+
+$(OBJ): all.h ops.h
+$(AMD64OBJ): amd64/all.h
+$(ARM64OBJ): arm64/all.h
+$(RV64OBJ): rv64/all.h
+main.o: config.h
+
+config.h:
+	@case `uname` in                               \
+	*Darwin*)                                      \
+		case `uname -m` in                     \
+		*arm64*)                               \
+			echo "#define Deftgt T_arm64_apple";\
+			;;                             \
+		*)                                     \
+			echo "#define Deftgt T_amd64_apple";\
+			;;                             \
+		esac                                   \
+		;;                                     \
+	*)                                             \
+		case `uname -m` in                     \
+		*aarch64*|*arm64*)                     \
+			echo "#define Deftgt T_arm64"; \
+			;;                             \
+		*riscv64*)                             \
+			echo "#define Deftgt T_rv64";  \
+			;;                             \
+		*)                                     \
+			echo "#define Deftgt T_amd64_sysv";\
+			;;                             \
+		esac                                   \
+		;;                                     \
+	esac > $@
+
+install: qbe
+	mkdir -p "$(DESTDIR)$(BINDIR)"
+	install -m755 qbe "$(DESTDIR)$(BINDIR)/qbe"
+
+uninstall:
+	rm -f "$(DESTDIR)$(BINDIR)/qbe"
+
+clean:
+	rm -f *.o */*.o qbe
+
+clean-gen: clean
+	rm -f config.h
+
+check: qbe
+	tools/test.sh all
+
+check-arm64: qbe
+	TARGET=arm64 tools/test.sh all
+
+check-rv64: qbe
+	TARGET=rv64 tools/test.sh all
+
+src:
+	@echo $(SRCALL)
+
+80:
+	@for F in $(SRCALL);                       \
+	do                                         \
+		awk "{                             \
+			gsub(/\\t/, \"        \"); \
+			if (length(\$$0) > $@)     \
+				printf(\"$$F:%d: %s\\n\", NR, \$$0); \
+		}" < $$F;                          \
+	done
+
+.PHONY: clean clean-gen check check-arm64 src 80 install uninstall