feat(inventory): add upc.h (Phase 1: scan + UPC validation)

code/inventory/upc.h

@@ -0,0 +1,24 @@
+/*
+ * upc.h - UPC-A barcode validation.
+ *
+ * Pure C, no SIBO/PLIB dependencies, so it can be unit-tested on any host
+ * compiler as well as built for the Workabout.
+ */
+#ifndef UPC_H
+#define UPC_H
+
+#define UPC_A_LEN 12
+
+/*
+ * Returns 1 if s is a valid 12-digit UPC-A (all digits and a correct
+ * check digit), 0 otherwise. s must be NUL-terminated.
+ */
+int upcIsValid(const char *s);
+
+/*
+ * Returns the UPC-A check digit (0-9) computed from the first 11 digits
+ * of s, or -1 if those 11 characters are not all digits.
+ */
+int upcCheckDigit(const char *s);
+
+#endif /* UPC_H */