test: add end-to-end integration across dash/bash/zsh

tests/integration/fixture/Makefile.in

@@ -0,0 +1,27 @@
+# Makefile.in - integration-fixture template (plan todo 26).
+#
+# The substitution placeholders are filled by the generated ./configure
+# (they must stay literal here - never expand them in this file). CC and
+# CFLAGS come from the C language module's detection, LIBS from the feature
+# probes' link flags (-lpthread/-lm), prefix from the --prefix default or
+# override.
+#
+# NOTE: the substitution is LINE-BASED and runs on every line, comments
+# included - so no comment may contain a placeholder-shaped token (the
+# configure script would treat it as a variable to substitute).
+#
+# This file is NOT generated by stupidtools: the tool only substitutes
+# placeholders into it (Makefile generation is out of v1 scope by design).
+
+CC = @CC@
+CFLAGS = @CFLAGS@ -std=c23 -Wall -Wextra -Wpedantic
+LIBS = @LIBS@
+prefix = @prefix@
+
+all: demo
+
+demo: main.c demo.c demo.h
+	$(CC) $(CFLAGS) main.c demo.c -o demo $(LIBS)
+
+clean:
+	rm -f demo