fix(chart): align OOXML builder palette with SVG preview palette

ChartHelper.Builder.DefaultSeriesColors and ChartSvgRenderer.FallbackColors
both held a 12-color default rotation but disagreed on slots 8/10/11/12 —
the darker (lumMod=75%) tints of accents 2/4/5/6. The Builder values
(9B4A22/BF8F00/3A75A8/4E8538) were near-but-wrong; the Renderer values
(9E480E/997300/255E91/43682B) match Office's actual lumMod=75% derivation.

Result: charts written with no explicit colors picked one palette while
the in-app HTML preview drew the same series with a slightly different
palette — a real visual drift.

Both arrays now derive from a single canonical source on
OfficeDefaultThemeColors.DefaultChartSeriesPalette.

Author: goworm

src/officecli/Core/Chart/ChartHelper.Builder.cs

@@ -820,11 +820,11 @@ internal static C.StockChart BuildStockChart(
 
     // ==================== Default Series Colors ====================
 
+    // CONSISTENCY(chart-default-palette): canonical source is
+    // OfficeDefaultThemeColors.DefaultChartSeriesPalette so the OOXML
+    // builder and the SVG preview renderer cannot drift apart.
     internal static readonly string[] DefaultSeriesColors =
-    {
-        "4472C4", "ED7D31", "A5A5A5", "FFC000", "5B9BD5", "70AD47",
-        "264478", "9B4A22", "636363", "BF8F00", "3A75A8", "4E8538"
-    };
+        OfficeDefaultThemeColors.DefaultChartSeriesPalette;
 
     // ==================== Series Color ====================
 

src/officecli/Core/Chart/ChartSvgRenderer.cs

@@ -17,11 +17,13 @@ namespace OfficeCli.Core;
 /// </summary>
 internal partial class ChartSvgRenderer
 {
-    // Fallback chart colors — used only when no theme is available
-    public static readonly string[] FallbackColors = [
-        "#4472C4", "#ED7D31", "#A5A5A5", "#FFC000", "#5B9BD5", "#70AD47",
-        "#264478", "#9E480E", "#636363", "#997300", "#255E91", "#43682B"
-    ];
+    // CONSISTENCY(chart-default-palette): canonical source is
+    // OfficeDefaultThemeColors.DefaultChartSeriesPalette; SVG just needs
+    // the '#'-prefixed form, so we derive once at static init.
+    public static readonly string[] FallbackColors =
+        OfficeDefaultThemeColors.DefaultChartSeriesPalette
+            .Select(hex => "#" + hex)
+            .ToArray();
 
     /// <summary>
     /// Theme-derived accent colors for chart series. Set from document theme accent1-6.

src/officecli/Core/OfficeDefaultThemeColors.cs

@@ -33,6 +33,22 @@ public static class OfficeDefaultThemeColors
     public const string Hyperlink = "0563C1";
     public const string FollowedHyperlink = "954F72";
 
+    /// <summary>
+    /// Default chart series color rotation when no <c>ColorScheme</c> is
+    /// available. Slots 1-6 are the six accent colors; slots 7-12 are the
+    /// same accents with <c>lumMod=75000</c> applied (the darker tints
+    /// Office cycles through after exhausting the primary accents).
+    ///
+    /// Hex values are 6-char OOXML format (no leading <c>#</c>). Both the
+    /// OOXML chart Builder and the SVG preview Renderer derive from this
+    /// array — keep them aligned to avoid the chart-vs-preview drift.
+    /// </summary>
+    public static readonly string[] DefaultChartSeriesPalette =
+    {
+        Accent1, Accent2, Accent3, Accent4, Accent5, Accent6,
+        "264478", "9E480E", "636363", "997300", "255E91", "43682B",
+    };
+
     /// <summary>
     /// Builds a name→hex dictionary covering the canonical scheme keys plus
     /// the common aliases (dk1/tx1/text1, bg1/lt1/background1, …) that Word