fix(io): omit removedClamps from sheets without a removal

Every plate was written with "removedClamps": [], so files without the
feature changed. The key is now omitted unless a clamp was removed, like
the other additive sections; nest-file-format.md documents removedClamps
and removalMinutes and their lenient reading.

PM 6ded8ac4-84fd-4e8b-9a3b-6f41e494542c item (c)

OpenNest.IO.Tests/ClampRailPersistenceTests.cs

@@ -236,8 +236,10 @@ public class ClampRailPersistenceTests
     [Fact]
     public void LegacyFileWithoutRemovedClampsKeyLoadsWithNoneRemoved()
     {
-        var withoutKey = RewriteNestJson(Write(RailNest()),
-            text => RemoveJsonProperty(text, "removedClamps"));
+        // Sheets without a removal are written without the key, exactly like a file from
+        // before the feature.
+        var withoutKey = Write(RailNest());
+        Assert.DoesNotContain("\"removedClamps\"", ReadNestJson(withoutKey), StringComparison.Ordinal);
 
         var reopened = new NestReader(withoutKey).Read();
 
@@ -309,6 +311,16 @@ public class ClampRailPersistenceTests
         Assert.NotNull(plate.ClampRail);
     }
 
+    // PM 6ded8ac4 (c): sheets without a removal write no removedClamps key at all.
+    [Fact]
+    public void SheetsWithoutRemovedClampsWriteNoRemovedClampsKey()
+    {
+        var json = ReadNestJson(Write(RailNest()));
+
+        Assert.DoesNotContain("\"removedClamps\"", json, StringComparison.Ordinal);
+        Assert.Contains("\"removedClamps\"", ReadNestJson(RemovedClamp3Nest()), StringComparison.Ordinal);
+    }
+
     /// <summary>Removes a named property wherever it appears in the document, simulating a
     /// key an older writer never emitted. Rebuilds via <see cref="JsonNode"/> so the result
     /// is always valid JSON regardless of whether the property was first, last or only one

OpenNest.IO/NestFormat.cs

@@ -206,11 +206,12 @@ namespace OpenNest.IO
             public List<ClampZoneDto> Clamps { get; init; } = new();
 
             /// <summary>
-            /// Rail clamp ids removed from this sheet as a last resort (D8). Missing or empty
-            /// means none were removed; <see cref="Clamps"/> already excludes them. Read
+            /// Rail clamp ids removed from this sheet as a last resort (D8). Omitted when none
+            /// were removed; missing or empty means none; <see cref="Clamps"/> already excludes them. Read
             /// leniently: a wrongly shaped value reads as none removed, non-string entries
             /// are skipped, and the saved zones load either way.
             /// </summary>
+            [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
             [JsonConverter(typeof(LenientStringListConverter))]
             public List<string> RemovedClamps { get; init; } = new();
         }

OpenNest.IO/NestWriter.cs

@@ -334,10 +334,14 @@ namespace OpenNest.IO
                                 Width = z.Bounds.Width,
                             })
                             .ToList(),
-                        RemovedClamps = plate.RemovedClamps
-                            .OrderBy(ClampIndex)
-                            .ThenBy(id => id, StringComparer.Ordinal)
-                            .ToList(),
+                        // Omitted unless a clamp was removed (files without the feature
+                        // stay unchanged).
+                        RemovedClamps = plate.RemovedClamps.Count == 0
+                            ? null
+                            : plate.RemovedClamps
+                                .OrderBy(ClampIndex)
+                                .ThenBy(id => id, StringComparer.Ordinal)
+                                .ToList(),
                     }
                 );
             }

docs/nest-file-format.md

@@ -80,6 +80,14 @@ malformed rail loads as none. On load a plate follows the rail again only when
 its saved `clamps` are exactly the rail's zones for that sheet; any other saved
 zones are kept as written.
 
+The rail's optional `removalMinutes` is the time to take one clamp off and put
+it back; a missing or invalid value loads as five minutes. A plate on which
+Auto Nest removed clamps as a last resort lists their rail ids in an optional
+`removedClamps` array (omitted when none were removed); its `clamps` already
+exclude them, and it follows the rail minus those ids. A malformed
+`removedClamps` value reads as none removed (non-string entries are skipped);
+the saved `clamps` load either way.
+
 An empty plate (no parts, no cut-offs) is omitted from the file unless that
 would lose clamp zones: it is omitted like any other empty plate only when a
 new plate from the saved defaults would recreate it (same size and quadrant,