normalizer: configurable float epsilon + JSON-path volatile mask scoping

Follow-ups to #4 normalizer PoC v2:
- Profile.float_decimals (default 6) flows into Rules.RoundFloatsInNode.
- mask_volatile_settings switches from name-only HashSet to a JSONPath-lite
  allowlist ($.a.b.c) so same-named fields in unrelated subtrees stay intact.
- default.yaml migrated; 6 new tests including a regression trap for the
  unrelated-subtree case. 16/16 normalizer tests, 77/77 solution tests.

Refs #2

tests/Recordingtest.Normalizer.Tests/RuleTests.cs

@@ -109,6 +109,89 @@ public class RuleTests
         }
     }
 
+    [Fact]
+    public void RoundFloats_DefaultProfile_Rounds6Decimals()
+    {
+        var input = "{\"x\": 3.1415926535897932}";
+        var r = Normalizer.Normalize(input, "default");
+        var node = JsonNode.Parse(r.Output)!;
+        Assert.Equal(3.141593, node["x"]!.GetValue<double>());
+    }
+
+    [Fact]
+    public void RoundFloats_ProfileWithDecimals3_RoundsTo3()
+    {
+        // Write a temp profile next to the loaded profiles dir.
+        var baseDir = AppContext.BaseDirectory;
+        var dest = Path.Combine(baseDir, "profiles", "test_decimals3.yaml");
+        File.WriteAllText(dest,
+            "name: test_decimals3\nfloat_decimals: 3\nrules:\n  - round_floats\n");
+        try
+        {
+            var input = "{\"x\": 3.1415926535}";
+            var r = Normalizer.Normalize(input, "test_decimals3");
+            var node = JsonNode.Parse(r.Output)!;
+            Assert.Equal(3.142, node["x"]!.GetValue<double>());
+        }
+        finally
+        {
+            if (File.Exists(dest)) File.Delete(dest);
+        }
+    }
+
+    [Fact]
+    public void Profile_OmittedFloatDecimals_DefaultsTo6()
+    {
+        var baseDir = AppContext.BaseDirectory;
+        var dest = Path.Combine(baseDir, "profiles", "test_no_decimals.yaml");
+        File.WriteAllText(dest, "name: test_no_decimals\nrules:\n  - round_floats\n");
+        try
+        {
+            var profile = Profile.Load("test_no_decimals");
+            Assert.Null(profile.FloatDecimals);
+            var input = "{\"x\": 3.1415926535897932}";
+            var r = Normalizer.Normalize(input, "test_no_decimals");
+            var node = JsonNode.Parse(r.Output)!;
+            Assert.Equal(3.141593, node["x"]!.GetValue<double>());
+        }
+        finally
+        {
+            if (File.Exists(dest)) File.Delete(dest);
+        }
+    }
+
+    [Fact]
+    public void MaskVolatileSettings_RootField_Masks()
+    {
+        var node = JsonNode.Parse("{\"GridSnap\":true,\"Other\":1}");
+        var (n, c) = Rules.MaskVolatileSettings(node, new[] { "$.GridSnap" });
+        Assert.Equal(1, c);
+        Assert.Equal("<VOLATILE>", n!["GridSnap"]!.GetValue<string>());
+        Assert.Equal(1, n!["Other"]!.GetValue<int>());
+    }
+
+    [Fact]
+    public void MaskVolatileSettings_SameNameInUnrelatedSubtree_NotMasked()
+    {
+        // Root has GridSnap (should mask), and an unrelated subtree Foo.GridSnap (should NOT mask).
+        var node = JsonNode.Parse("{\"GridSnap\":true,\"Foo\":{\"GridSnap\":false}}");
+        var (n, c) = Rules.MaskVolatileSettings(node, new[] { "$.GridSnap" });
+        Assert.Equal(1, c);
+        Assert.Equal("<VOLATILE>", n!["GridSnap"]!.GetValue<string>());
+        // Unrelated subtree must remain its original boolean value.
+        Assert.False(n!["Foo"]!["GridSnap"]!.GetValue<bool>());
+    }
+
+    [Fact]
+    public void MaskVolatileSettings_NestedPath_MasksCorrectly()
+    {
+        var node = JsonNode.Parse("{\"GridColor\":{\"R\":12,\"G\":34}}");
+        var (n, c) = Rules.MaskVolatileSettings(node, new[] { "$.GridColor.R" });
+        Assert.Equal(1, c);
+        Assert.Equal("<VOLATILE>", n!["GridColor"]!["R"]!.GetValue<string>());
+        Assert.Equal(34, n!["GridColor"]!["G"]!.GetValue<int>());
+    }
+
     [Fact]
     public void Normalize_SidecarPath_AcceptsDirectory()
     {