perf: cache SharedString table as O(1) List on first access

src/officecli/Core/HtmlScreenshot.cs

@@ -411,7 +411,7 @@ private static (bool, string?) TryChrome(string url, string outPath, int w, int
 
     private static string? FindChrome()
     {
-        string[] names = ["google-chrome", "google-chrome-stable", "chromium", "chromium-browser",
+        string[] names = ["chrome-headless-shell", "google-chrome", "google-chrome-stable", "chromium", "chromium-browser",
                           "chrome", "microsoft-edge", "microsoft-edge-stable", "msedge"];
         var pathHit = WhichFirst(names);
         if (pathHit != null) return pathHit;
@@ -437,12 +437,32 @@ private static (bool, string?) TryChrome(string url, string outPath, int w, int
         }
         else if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
         {
+            string localAppData = Environment.GetEnvironmentVariable("LOCALAPPDATA") ?? "";
+            string userProfile = Environment.GetFolderPath(Environment.SpecialFolder.UserProfile);
+
+            // Search ms-playwright / puppeteer installation directories for chrome-headless-shell.exe
+            foreach (var baseDir in new[] { localAppData, userProfile })
+            {
+                if (string.IsNullOrEmpty(baseDir)) continue;
+                var pwDir = Path.Combine(baseDir, "ms-playwright");
+                if (Directory.Exists(pwDir))
+                {
+                    try
+                    {
+                        foreach (var exe in Directory.GetFiles(pwDir, "chrome-headless-shell.exe", SearchOption.AllDirectories))
+                            abs.Add(exe);
+                    }
+                    catch { }
+                }
+            }
+
             string[] roots = [
                 Environment.GetEnvironmentVariable("PROGRAMFILES") ?? @"C:\Program Files",
                 Environment.GetEnvironmentVariable("PROGRAMFILES(X86)") ?? @"C:\Program Files (x86)",
-                Environment.GetEnvironmentVariable("LOCALAPPDATA") ?? "",
+                localAppData,
             ];
             string[] suffixes = [
+                @"Google\Chrome\Application\chrome-headless-shell.exe",
                 @"Google\Chrome\Application\chrome.exe",
                 @"Chromium\Application\chrome.exe",
                 @"Microsoft\Edge\Application\msedge.exe",

src/officecli/Handlers/Excel/ExcelHandler.Add.Cells.cs

@@ -1245,6 +1245,7 @@ private string AddRun(string parentPath, string type, InsertPosition? position,
                     new Text(runExistingText) { Space = SpaceProcessingModeValues.Preserve }));
             runCell.RemoveAllChildren<InlineString>();
             runSst.AppendChild(runSsi);
+            InvalidateSharedStringCache();
             var newSstIdx = runSst.Elements<SharedStringItem>().Count() - 1;
             runCell.CellValue = new CellValue(newSstIdx.ToString());
             runCell.DataType = new EnumValue<CellValues>(CellValues.SharedString);
@@ -1563,6 +1564,7 @@ private void ApplyRichTextToCell(Cell cell, Dictionary<string, string> propertie
         }
 
         sst.AppendChild(ssi);
+        InvalidateSharedStringCache();
         sst.Count = (uint)sst.Elements<SharedStringItem>().Count();
         sst.UniqueCount = sst.Count;
 
@@ -1621,6 +1623,7 @@ private void ApplyPhoneticToCell(Cell cell, WorksheetPart wsPart,
         ssi.AppendChild(rPh);
 
         sst.AppendChild(ssi);
+        InvalidateSharedStringCache();
         sst.Count = (uint)sst.Elements<SharedStringItem>().Count();
         sst.UniqueCount = sst.Count;
 

src/officecli/Handlers/Excel/ExcelHandler.Helpers.Cell.cs

@@ -38,6 +38,48 @@ public partial class ExcelHandler
             $"Unknown totals-row function '{tok}'. Valid: sum, average, count, countNums, max, min, stdDev, var, none, custom.")
     };
 
+    private List<SharedStringItem>? _sharedStringItemCache;
+    private List<string>? _sharedStringCache;
+
+    private List<SharedStringItem> GetSharedStringItemCache()
+    {
+        if (_sharedStringItemCache != null) return _sharedStringItemCache;
+        var sst = _doc.WorkbookPart?.GetPartsOfType<SharedStringTablePart>().FirstOrDefault();
+        if (sst?.SharedStringTable == null) return _sharedStringItemCache = new List<SharedStringItem>();
+
+        var list = new List<SharedStringItem>();
+        foreach (var item in sst.SharedStringTable.Elements<SharedStringItem>())
+        {
+            list.Add(item);
+        }
+        return _sharedStringItemCache = list;
+    }
+
+    private List<string> GetSharedStringCache()
+    {
+        if (_sharedStringCache != null) return _sharedStringCache;
+        var items = GetSharedStringItemCache();
+        var list = new List<string>(items.Count);
+        foreach (var item in items)
+        {
+            list.Add(item.InnerText ?? "");
+        }
+        return _sharedStringCache = list;
+    }
+
+    /// <summary>
+    /// Must be called whenever a new SharedStringItem is appended to or removed
+    /// from the SharedStringTable so that the next GetCellDisplayValue call
+    /// rebuilds the cache from the updated table.
+    /// </summary>
+    private void InvalidateSharedStringCache()
+    {
+        _sharedStringCache = null;
+        _sharedStringItemCache = null;
+    }
+
+
+
     private string GetCellDisplayValue(Cell cell, Core.FormulaEvaluator? evaluator = null)
     {
         if (cell.DataType?.Value == CellValues.InlineString)
@@ -49,12 +91,13 @@ private string GetCellDisplayValue(Cell cell, Core.FormulaEvaluator? evaluator =
 
         if (cell.DataType?.Value == CellValues.SharedString)
         {
-            var sst = _doc.WorkbookPart?.GetPartsOfType<SharedStringTablePart>().FirstOrDefault();
-            if (sst?.SharedStringTable != null && int.TryParse(value, out int idx))
+            if (int.TryParse(value, out int idx))
             {
-                var item = sst.SharedStringTable.Elements<SharedStringItem>().ElementAtOrDefault(idx);
-                return item?.InnerText ?? value;
+                var cache = GetSharedStringCache();
+                if (idx >= 0 && idx < cache.Count)
+                    return cache[idx];
             }
+            return value;
         }
 
         // Boolean cells store 0/1 in <v> per the OOXML spec, but Excel displays

src/officecli/Handlers/Excel/ExcelHandler.HtmlPreview.cs

@@ -2935,9 +2935,10 @@ private static (double mantissa, int exp) NormalizeScientific(double value)
         var value = cell.CellValue?.Text;
         if (value == null || !int.TryParse(value, out int idx)) return null;
 
-        var sst = _doc.WorkbookPart?.GetPartsOfType<SharedStringTablePart>().FirstOrDefault();
-        var item = sst?.SharedStringTable?.Elements<SharedStringItem>().ElementAtOrDefault(idx);
-        if (item == null) return null;
+        var items = GetSharedStringItemCache();
+        if (idx < 0 || idx >= items.Count) return null;
+        var item = items[idx];
+
 
         var runs = item.Elements<Run>().ToList();
         // Only worth wrapping when at least one run carries explicit run-properties;