Invoice set pricing: 3 display modes (backend contract + PDF + tests)

Sets (mfr__items Type='set') can be shown three ways on the invoice:
- SetPrice (default): set line priced, member items shown without price
- ItemPrices: member items priced, set line as a heading without price
- SetOnly: only the set line (priced), members removed

- InvoiceSetPricing (new): the authoritative, unit-tested transformation
  (SetDisplayMode + Build) that both sides agree on; set price always equals the
  sum of members. Mode is read from InvoiceOptions ("setmode:<mode>").
- FuchsPdf.ApplyInvoice renders through it: lines flagged ShowPrice=false print
  blank price/total cells; set headers are emphasised. Invoices without sets are
  unchanged. Totals come from the registration balance, so modes are purely
  presentational and never change the sum.
- InvoiceSetPricingTests (+14): all three modes, set-price = member sum, header
  total fallback, no-set pass-through, option parsing.
- Docs/INVOICE_SET_PRICING.md documents the front-end contract (the editor sets
  the mode token + tags set header/member items); the back-end does the rest.

Front-end editor wiring is specified in the doc but intentionally not shipped
blind (cannot validate the running editor here).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Fuchs.Tests/InvoiceSetPricingTests.cs

@@ -0,0 +1,157 @@
+using System.Collections.Generic;
+using System.Linq;
+using Fuchs.intranet;
+using Xunit;
+
+namespace Fuchs.Tests;
+
+/// <summary>
+/// Contract tests for the set-pricing display modes — the agreed interplay
+/// between the front-end (which picks the mode and groups items) and the
+/// back-end PDF renderer (which honours ShowPrice / IsSetHeader). Set items use
+/// type="set"; members carry setId == the set header's id.
+/// </summary>
+public class InvoiceSetPricingTests
+{
+    private static Dictionary<string, object?> SetHeader(string id, string title, string? total = null) => new()
+    {
+        ["type"] = "set", ["id"] = id, ["title"] = title, ["qty"] = "1",
+        ["total_net"] = total ?? "0", ["price_net"] = total ?? "0"
+    };
+
+    private static Dictionary<string, object?> Member(string setId, string title, string price, string total) => new()
+    {
+        ["setId"] = setId, ["title"] = title, ["qty"] = "1", ["price_net"] = price, ["total_net"] = total
+    };
+
+    private static Dictionary<string, object?> Standalone(string title, string price, string total) => new()
+    {
+        ["title"] = title, ["qty"] = "1", ["price_net"] = price, ["total_net"] = total
+    };
+
+    // A set (sum 1000) with two members, plus one standalone item (50).
+    private static List<Dictionary<string, object?>> Sample() => new()
+    {
+        SetHeader("10", "Bad-Komplettset", "1000.00"),
+        Member("10", "Waschbecken", "600.00", "600.00"),
+        Member("10", "Armatur",     "400.00", "400.00"),
+        Standalone("Anfahrt", "50.00", "50.00")
+    };
+
+    // ── Mode parsing ────────────────────────────────────────────────────────
+    [Theory]
+    [InlineData("itemprices", SetDisplayMode.ItemPrices)]
+    [InlineData("items",      SetDisplayMode.ItemPrices)]
+    [InlineData("setonly",    SetDisplayMode.SetOnly)]
+    [InlineData("setprice",   SetDisplayMode.SetPrice)]
+    [InlineData("",           SetDisplayMode.SetPrice)]
+    [InlineData("garbage",    SetDisplayMode.SetPrice)]
+    public void ParseMode_Works(string raw, SetDisplayMode expected)
+        => Assert.Equal(expected, InvoiceSetPricing.ParseMode(raw));
+
+    [Fact]
+    public void ModeFromInvoiceOptions_ReadsToken()
+    {
+        Assert.Equal(SetDisplayMode.ItemPrices, InvoiceSetPricing.ModeFromInvoiceOptions("§13b,setmode:itemprices"));
+        Assert.Equal(SetDisplayMode.SetOnly,    InvoiceSetPricing.ModeFromInvoiceOptions("setmode:setonly"));
+        Assert.Equal(SetDisplayMode.SetPrice,   InvoiceSetPricing.ModeFromInvoiceOptions("§13b"));      // default
+        Assert.Equal(SetDisplayMode.SetPrice,   InvoiceSetPricing.ModeFromInvoiceOptions(null));
+    }
+
+    [Fact]
+    public void ContainsSets_DetectsHeaderOrMember()
+    {
+        Assert.True(InvoiceSetPricing.ContainsSets(Sample()));
+        Assert.False(InvoiceSetPricing.ContainsSets(new List<Dictionary<string, object?>> { Standalone("X", "1", "1") }));
+    }
+
+    // ── SetPrice (default): set priced, members blank ─────────────────────────
+    [Fact]
+    public void Build_SetPrice_SetLinePricedMembersBlank()
+    {
+        var lines = InvoiceSetPricing.Build(Sample(), SetDisplayMode.SetPrice);
+
+        Assert.Equal(4, lines.Count);                       // header + 2 members + standalone
+        var header = lines[0];
+        Assert.True(header.IsSetHeader);
+        Assert.True(header.ShowPrice);
+        Assert.Equal(1000.00m, header.TotalNet);
+
+        Assert.False(lines[1].ShowPrice);                   // Waschbecken — no price
+        Assert.False(lines[2].ShowPrice);                   // Armatur — no price
+        Assert.Equal("Waschbecken", lines[1].Title);
+
+        Assert.True(lines[3].ShowPrice);                    // standalone keeps price
+        Assert.Equal(50.00m, lines[3].TotalNet);
+    }
+
+    // ── ItemPrices: members priced, set header blank ──────────────────────────
+    [Fact]
+    public void Build_ItemPrices_MembersPricedHeaderBlank()
+    {
+        var lines = InvoiceSetPricing.Build(Sample(), SetDisplayMode.ItemPrices);
+
+        Assert.Equal(4, lines.Count);
+        Assert.True(lines[0].IsSetHeader);
+        Assert.False(lines[0].ShowPrice);                   // set header is just a title now
+        Assert.True(lines[1].ShowPrice);
+        Assert.Equal(600.00m, lines[1].TotalNet);
+        Assert.True(lines[2].ShowPrice);
+        Assert.Equal(400.00m, lines[2].TotalNet);
+        Assert.True(lines[3].ShowPrice);                    // standalone
+    }
+
+    // ── SetOnly: members removed ──────────────────────────────────────────────
+    [Fact]
+    public void Build_SetOnly_RemovesMembers()
+    {
+        var lines = InvoiceSetPricing.Build(Sample(), SetDisplayMode.SetOnly);
+
+        Assert.Equal(2, lines.Count);                       // set header + standalone only
+        Assert.True(lines[0].IsSetHeader);
+        Assert.True(lines[0].ShowPrice);
+        Assert.Equal(1000.00m, lines[0].TotalNet);
+        Assert.False(lines[1].IsSetHeader);
+        Assert.Equal("Anfahrt", lines[1].Title);
+    }
+
+    // ── Set price falls back to sum of members when header total is 0 ─────────
+    [Fact]
+    public void Build_HeaderTotalZero_UsesSumOfMembers()
+    {
+        var items = new List<Dictionary<string, object?>>
+        {
+            SetHeader("7", "Set ohne Preis"),               // total 0
+            Member("7", "A", "120.00", "120.00"),
+            Member("7", "B", "80.00",  "80.00")
+        };
+        var lines = InvoiceSetPricing.Build(items, SetDisplayMode.SetPrice);
+        Assert.Equal(200.00m, lines[0].TotalNet);           // 120 + 80
+    }
+
+    // ── No sets: pass-through unchanged ───────────────────────────────────────
+    [Fact]
+    public void Build_NoSets_PassThroughAllPriced()
+    {
+        var items = new List<Dictionary<string, object?>>
+        {
+            Standalone("A", "10.00", "10.00"),
+            Standalone("B", "20.00", "20.00")
+        };
+        var lines = InvoiceSetPricing.Build(items, SetDisplayMode.SetPrice);
+        Assert.Equal(2, lines.Count);
+        Assert.All(lines, l => Assert.True(l.ShowPrice));
+        Assert.All(lines, l => Assert.False(l.IsSetHeader));
+    }
+
+    // ── Set price equals sum of member prices across modes (no double counting) ─
+    [Fact]
+    public void SetPrice_EqualsSumOfMembers()
+    {
+        var setLine = InvoiceSetPricing.Build(Sample(), SetDisplayMode.SetPrice).First(l => l.IsSetHeader);
+        decimal memberSum = Sample()
+            .Where(i => i.TryGetValue("setId", out var s) && s?.ToString() == "10")
+            .Sum(i => decimal.Parse(i["total_net"]!.ToString()!, System.Globalization.CultureInfo.InvariantCulture));
+        Assert.Equal(memberSum, setLine.TotalNet);
+    }
+}