Rebuild the Classic template on the engine with A4/Letter and a template harness

Classic now renders through buildRenderModel, computeFrame, the template
and renderCore, with S.No and HSN/SAC columns, GST slots only through
checked Slot components, taxable value, reverse-charge line, cancelled tag,
continuation band and Page n of N. Letter absorbs its height deficit in
elastic gaps. A harness runs 15 fixtures on A4 and Letter plus an item
sweep and asserts zero audit errors, repeated headers, no split blocks,
rupee extraction and deterministic output. The legacy InvoiceDocument is
removed.

src/pdf/blocks/Row.tsx

@@ -21,7 +21,7 @@ export interface RowProps extends PdfFlagsInput {
 }
 
 /**
- * A flex row whose children each sit in a cell of explicit width: fixed and fractional cells are pinned
+ * A flex row, pinned to its total width (a row inside a flex-start parent would otherwise shrink to its content and leave its fill cell empty), whose children each sit in a cell of explicit width: fixed and fractional cells are pinned
  * (width = minWidth = maxWidth) and the fill cell uses {flexGrow: 1, flexBasis: 0}. Descendants read the
  * cell's content width through useAvailableWidth(). When a column falls under its `min` (or the row is
  * narrower than `stackBelow`), the children are stacked at full width instead.
@@ -38,7 +38,7 @@ export function Row({ id, widths, gap = 0, stackBelow, align = "flex-start", wid
 
   if (stacked) {
     return (
-      <View style={{ flexDirection: "column", alignItems: "flex-start", ...style } as never} {...flags}>
+      <View style={{ flexDirection: "column", alignItems: "flex-start", ...pinned(total), ...style } as never} {...flags}>
         {kids.map((child, i) => (
           <View key={i} style={{ ...pinned(total), alignItems: "flex-start", ...(gap && i > 0 ? { marginTop: gap } : {}) } as never}>
             <AvailableWidthProvider width={total}>{child}</AvailableWidthProvider>
@@ -49,7 +49,7 @@ export function Row({ id, widths, gap = 0, stackBelow, align = "flex-start", wid
   }
 
   return (
-    <View style={{ flexDirection: "row", alignItems: align, ...style } as never} {...flags}>
+    <View style={{ flexDirection: "row", alignItems: align, ...pinned(total), ...style } as never} {...flags}>
       {kids.map((child, i) => {
         const isFill = "fill" in widths[i];
         const box = isFill ? FILL : pinned(w[i]);