Add pdf-invoice module: one invoice in OpenPDF 3.0.5, iText Core 9.8.0 and Flying Saucer 10.7.0, Devanagari shaping checked by rendering, streamed vs buffered download, subset sizes and timings

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_017ic4b6Kg9RWSTJzyz6KP9X

pdf-invoice/src/main/java/com/ankurm/pdfinvoice/Invoice.java

@@ -0,0 +1,37 @@
+package com.ankurm.pdfinvoice;
+
+import java.math.BigDecimal;
+import java.math.RoundingMode;
+import java.time.LocalDate;
+import java.util.List;
+
+/**
+ * The one invoice all three libraries render. Plain records, no framework types, so each writer
+ * is a function {@code Invoice -> bytes} and the comparison is about the PDF libraries, not about Spring.
+ */
+public record Invoice(String number, LocalDate date, Party seller, Party buyer, List<Line> lines, String note) {
+
+    public record Party(String name, String address, String gstin) {}
+
+    /** {@code description} may be Devanagari; that is the point of the exercise. */
+    public record Line(String description, int quantity, BigDecimal unitPrice) {
+        public BigDecimal amount() {
+            return unitPrice.multiply(BigDecimal.valueOf(quantity));
+        }
+    }
+
+    /** GST at 18 percent, split evenly into CGST and SGST as an intra-state sale would be. */
+    public static final BigDecimal GST_RATE = new BigDecimal("0.18");
+
+    public BigDecimal subtotal() {
+        return lines.stream().map(Line::amount).reduce(BigDecimal.ZERO, BigDecimal::add);
+    }
+
+    public BigDecimal gst() {
+        return subtotal().multiply(GST_RATE).setScale(2, RoundingMode.HALF_UP);
+    }
+
+    public BigDecimal total() {
+        return subtotal().add(gst());
+    }
+}