fix: preserve legacy p-rec-annot elevation quotient formatting

Plant3D2026/P_Rec_Annot/src/PRecAnnot.Core/LegacyElevationFormatter.cs

@@ -6,8 +6,11 @@ public static class LegacyElevationFormatter
 {
     public static int RoundModelElevation(double modelZ, double datumOffset)
     {
-        var increments = Math.Round((modelZ + datumOffset) / 5d, MidpointRounding.AwayFromZero);
-        return checked((int)Math.Round(increments * 5d, MidpointRounding.AwayFromZero));
+        // InsertElevation.BlockJig divides the model-space ordinate by five,
+        // rounds that quotient away from zero, then formats the resulting
+        // integer as a signed metres/millimetres value.  It does not multiply
+        // the rounded quotient back by five.
+        return checked((int)Math.Round((modelZ + datumOffset) / 5d, MidpointRounding.AwayFromZero));
     }
 
     public static string FormatMillimetres(int millimetres)

Plant3D2026/P_Rec_Annot/tests/PRecAnnot.Core.Tests/ElevationAnnotationPlannerTests.cs

@@ -10,7 +10,7 @@ public sealed class ElevationAnnotationPlannerTests
     {
         var a = new ElevationCandidate("a", 5, 0, new(0, 0), 0, "PXP", "GOSTC");
         var b = new ElevationCandidate("b", 10, 0, new(1, 0), 0, "PXP", "GOSTC");
-        var plan = new ElevationAnnotationPlanner().Build([b, a, a], [new("old-a", "a", "+0,005", new(0, 0)), new("old-b1", "b", "+0,010", new()), new("old-b2", "b", "+0,010", new())]);
+        var plan = new ElevationAnnotationPlanner().Build([b, a, a], [new("old-a", "a", "+0,001", new(0, 0)), new("old-b1", "b", "+0,002", new()), new("old-b2", "b", "+0,002", new())]);
         Assert.Collection(plan.Operations,
             value => Assert.Equal(AnnotationOperationKind.Skip, value.Kind),
             value => Assert.Equal(AnnotationOperationKind.Conflict, value.Kind),
@@ -23,6 +23,6 @@ public sealed class ElevationAnnotationPlannerTests
     {
         var plan = new ElevationAnnotationPlanner().Build([new ElevationCandidate("a", 13, 0, new(), 0, "PXP", "GOSTC")], [new("old", "a", "+0,010", new())]);
         Assert.Equal(AnnotationOperationKind.Update, plan.Operations.Single().Kind);
-        Assert.Equal("+0,015", plan.Operations.Single().Text);
+        Assert.Equal("+0,003", plan.Operations.Single().Text);
     }
 }

Plant3D2026/P_Rec_Annot/tests/PRecAnnot.Core.Tests/LegacyElevationFormatterTests.cs

@@ -7,10 +7,10 @@ public sealed class LegacyElevationFormatterTests
     [Theory]
     [InlineData(0, 0, "0,000")]
     [InlineData(2.49, 0, "0,000")]
-    [InlineData(2.5, 0, "+0,005")]
-    [InlineData(-2.5, 0, "-0,005")]
-    [InlineData(1233, 2, "+1,235")]
-    [InlineData(-1233, -2, "-1,235")]
+    [InlineData(2.5, 0, "+0,001")]
+    [InlineData(-2.5, 0, "-0,001")]
+    [InlineData(1233, 2, "+0,247")]
+    [InlineData(-1233, -2, "-0,247")]
     public void PreservesFiveMillimetreAwayFromZeroLegacyRule(double modelZ, double datum, string expected) =>
         Assert.Equal(expected, LegacyElevationFormatter.Calculate(modelZ, datum));