fix(vtank): slice 7 fix round C item D3 — bottom-band pin counts implicit widget height

AssertWithinParent only flagged a child crossing a group's bottom edge
when that child declared its own h attribute — Number(child, "h") reads
0 for an absent attribute, so an unsized <label>/<field>/<toggle>/
<button> positioned right against a group's bottom edge silently passed
even though the retained-UI runtime still gives it a real default row
height at draw time. Added EffectiveHeight: falls back to each widget
kind's own implicit default when h is absent (label/field 16, toggle 20,
button 16 as a defensive floor since real buttons in this markup range
16-25px and always declare h explicitly); <list>/<menu> keep the old
"0 when absent" behavior since they have no implicit default at all.

Mutation named: temporarily reverted the check back to Number(child, "h")
and confirmed the new AssertWithinParent_CatchesAnUnsizedLabelNearTheBottomEdge
pin fails (no exception thrown for a synthetic <label y="190"> with no h
inside a 194-tall group, which should clip 12px past the bottom once the
16px default is counted) before restoring the fix. Audited every real
mosstank*.xml file for label/toggle/field/button elements missing h
(grep for each without ` h="`) — none exist, so this stricter check
introduces no new failures against the shipped markup.

MossTank suite 679 -> 680 (one new pin). Full solution build green; App
markup/plugin filter 203/203.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tests/AcDream.Plugins.MossTank.Tests/MossTankMarkupContractTests.cs

@@ -331,6 +331,26 @@ public sealed class MossTankMarkupContractTests
         AssertWithinParent(root);
     }
 
+    /// <summary>
+    /// Fix round C item D3: direct pin on <see cref="AssertWithinParent"/>'s
+    /// height check, independent of any real markup file. A
+    /// <c>&lt;label y="190"&gt;</c> with no declared <c>h</c> inside a
+    /// 194-tall group clips 12px past the bottom edge once the label's
+    /// real 16px default row height is counted (190 + 16 = 206 > 194) —
+    /// before this fix, an absent <c>h</c> read as zero height and the
+    /// check silently passed.
+    /// </summary>
+    [Fact]
+    public void AssertWithinParent_CatchesAnUnsizedLabelNearTheBottomEdge()
+    {
+        var group = new XElement("group",
+            new XAttribute("w", "848"), new XAttribute("h", "194"),
+            new XElement("label", new XAttribute("x", "4"), new XAttribute("y", "190"),
+                new XAttribute("text", "Notice")));
+
+        Assert.Throws<Xunit.Sdk.TrueException>(() => AssertWithinParent(group));
+    }
+
     /// <summary>
     /// Fix round B item 2: <see cref="AssertWithinParent"/> only ever proved
     /// a control fits inside its OWN parent's declared bounds — it never
@@ -571,7 +591,7 @@ public sealed class MossTankMarkupContractTests
         foreach (XElement child in parent.Elements())
         {
             float width = Number(child, "w");
-            float height = Number(child, "h");
+            float height = EffectiveHeight(child);
             if (width > 0f)
             {
                 Assert.True(
@@ -588,6 +608,38 @@ public sealed class MossTankMarkupContractTests
         }
     }
 
+    /// <summary>
+    /// Fix round C item D3: an element's declared <c>h</c> only tells the
+    /// whole story for widget kinds that never rely on an implicit
+    /// default. <c>&lt;label&gt;</c>/<c>&lt;field&gt;</c> without <c>h</c>
+    /// still occupy one DAT-font text row (16px) at runtime, and
+    /// <c>&lt;toggle&gt;</c> still occupies the checkbox lamp's own 20px
+    /// footprint — <see cref="AssertWithinParent"/> used to treat any of
+    /// those as zero-height (h absent) and silently pass an unsized notice
+    /// label positioned right up against a group's bottom edge, which
+    /// actually clips in the running client. <c>&lt;button&gt;</c> has no
+    /// single canonical default (real buttons in this markup range from
+    /// 16 to 25px and always declare h explicitly), so 16 — the same
+    /// floor as label/field — is used defensively; <c>&lt;list&gt;</c>/
+    /// <c>&lt;menu&gt;</c> have no implicit default at all and keep the
+    /// old "0 when absent" behavior.
+    /// </summary>
+    private static float EffectiveHeight(XElement element)
+    {
+        float declared = Number(element, "h");
+        if (declared > 0f)
+            return declared;
+
+        return element.Name.LocalName switch
+        {
+            "label" => 16f,
+            "field" => 16f,
+            "toggle" => 20f,
+            "button" => 16f,
+            _ => 0f,
+        };
+    }
+
     private static float Number(XElement element, string attribute) =>
         float.TryParse(
             (string?)element.Attribute(attribute),