Fix EPO control 1411 by deriving kc_zd6 from rounded prij6/danZah

Independent ToWholeCZK truncation of Section6GrossIncome, Section6ForeignTax
and Section6TaxBase could produce kc_zd6 != kc_prij6 - kc_dan_zah at whole-CZK
precision when source amounts had fractional CZK, breaking the EPO server-side
check "Oddíl 2/ř.36 - hodnota položky se nerovná hodnotě ř.34". Compute zd6
from the already-rounded prij6/danZah so the invariant holds for every input.

Author: zajca

internal/annualtaxxml/income_tax_gen.go

@@ -209,13 +209,15 @@ func GenerateIncomeTaxXML(itr *domain.IncomeTaxReturn, settings map[string]strin
 	uhrn := zd7 + zd8 + zd10                // ř.41 (no §9 rental income tracked yet)
 
 	// §6 employment income inputs.
-	// kc_zd6 (ř.34/36) is computed by the service as kc_prij6 - kc_dan_zah and stored
-	// in itr.Section6TaxBase. We carry the precomputed value through to keep XML and
-	// detail-page rendering consistent.
+	// EPO control 1411 ("Oddíl 2/ř.36 - hodnota položky se nerovná hodnotě ř.34")
+	// requires kc_zd6 == kc_prij6 - kc_dan_zah at whole-CZK precision. Computing
+	// zd6 from the already-rounded prij6/danZah (instead of rounding the halere
+	// Section6TaxBase independently) guarantees the equality even when the source
+	// amounts have fractional CZK that would otherwise truncate inconsistently.
 	prij6 := ToWholeCZK(itr.Section6GrossIncome)              // ř.31
 	prij6zahr := ToWholeCZK(itr.Section6IncomeWithoutAdvance) // ř.35
 	danZah := ToWholeCZK(itr.Section6ForeignTax)              // ř.33
-	zd6 := ToWholeCZK(itr.Section6TaxBase)                    // ř.34/36
+	zd6 := prij6 - danZah                                     // ř.34/36 = ř.31 - ř.33
 
 	// XSD ř.42 critical control: "Pokud je ř.41 záporný, uveďte pouze hodnotu z ř.36"
 	// — when §7+§8+§10 sum to a loss, the consolidated tax base is just §6 alone.

internal/annualtaxxml/income_tax_gen_test.go

@@ -741,6 +741,39 @@ func TestIncomeTaxXML_Section6Withholding(t *testing.T) {
 	}
 }
 
+// TestIncomeTaxXML_Section6Control1411 verifies that EPO control 1411
+// ("Oddíl 2/ř.36 - hodnota položky se nerovná hodnotě ř.34 z 2.oddílu DAP")
+// holds when the source amounts have fractional CZK (halere). EPO computes
+// ř.34 as kc_prij6 - kc_dan_zah at whole-CZK precision and requires kc_zd6
+// to match exactly; rounding each halere value independently can break the
+// invariant, so the generator derives kc_zd6 from the rounded prij6/danZah.
+func TestIncomeTaxXML_Section6Control1411(t *testing.T) {
+	// 240001.00 CZK gross, 1.50 CZK foreign tax: independent rounding of the
+	// stored Section6TaxBase (239999.50 CZK -> 239999) would not equal
+	// 240001 - 1 = 240000.
+	itr := &domain.IncomeTaxReturn{
+		Year:                2025,
+		FilingType:          domain.FilingTypeRegular,
+		TotalRevenue:        domain.NewAmount(0, 0),
+		Section6GrossIncome: domain.NewAmount(240001, 0),  // 240001.00 CZK
+		Section6ForeignTax:  domain.NewAmount(1, 50),      // 1.50 CZK
+		Section6TaxBase:     domain.NewAmount(239999, 50), // 239999.50 CZK
+	}
+	xmlData, err := GenerateIncomeTaxXML(itr, section6BaseSettings(), nil)
+	if err != nil {
+		t.Fatalf("GenerateIncomeTaxXML: %v", err)
+	}
+	for _, want := range []string{
+		`kc_prij6="240001"`,
+		`kc_dan_zah="1"`,
+		`kc_zd6="240000"`, // = kc_prij6 - kc_dan_zah, satisfies EPO control 1411
+	} {
+		if !bytes.Contains(xmlData, []byte(want)) {
+			t.Errorf("expected XML to contain %q, got:\n%s", want, xmlData)
+		}
+	}
+}
+
 // TestIncomeTaxXML_Section6OnlyNegativeSection7 covers the XSD critical control on
 // kc_zakldan23 (ř.42): "Pokud je ř.41 záporný, uveďte pouze hodnotu z ř.36". When
 // the §7+§8+§10 sum is negative, the consolidated tax base equals §6 alone.