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

tests/integration/fixture/demo.c

@@ -0,0 +1,16 @@
+/*
+ * demo.c - second translation unit of the integration fixture.
+ *
+ * Gives the fixture a real multi-file shape (the header demo.h is part of
+ * the deliverable "sources + headers"). Deliberately libm-free: the only
+ * libm dependency lives in main.c (sin), so a broken -lm accumulation is
+ * caught at LINK time there.
+ */
+
+#include "demo.h"
+
+int
+demo_compute(int x)
+{
+    return x * 3 + 1;
+}