feat: port p-rec-annot elevation formatting foundation

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

@@ -0,0 +1,23 @@
+using System.Globalization;
+
+namespace PRecAnnot.Core;
+
+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));
+    }
+
+    public static string FormatMillimetres(int millimetres)
+    {
+        if (millimetres == 0) return "0,000";
+        var magnitude = Math.Abs((long)millimetres).ToString(CultureInfo.InvariantCulture).PadLeft(4, '0');
+        var metres = magnitude[..^3];
+        var fraction = magnitude[^3..];
+        return $"{(millimetres > 0 ? "+" : "-")}{metres},{fraction}";
+    }
+
+    public static string Calculate(double modelZ, double datumOffset) => FormatMillimetres(RoundModelElevation(modelZ, datumOffset));
+}

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

@@ -0,0 +1,25 @@
+using PRecAnnot.Core;
+
+namespace PRecAnnot.Core.Tests;
+
+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")]
+    public void PreservesFiveMillimetreAwayFromZeroLegacyRule(double modelZ, double datum, string expected) =>
+        Assert.Equal(expected, LegacyElevationFormatter.Calculate(modelZ, datum));
+
+    [Theory]
+    [InlineData(5, "+0,005")]
+    [InlineData(50, "+0,050")]
+    [InlineData(500, "+0,500")]
+    [InlineData(1000, "+1,000")]
+    [InlineData(-5, "-0,005")]
+    public void FormatsSignedMetresFromLegacyMillimetreInteger(int value, string expected) =>
+        Assert.Equal(expected, LegacyElevationFormatter.FormatMillimetres(value));
+}