fix(localization): make locale-parity test able to detect a malformed locale file

LocaleStore.Load silently skips a *.json that fails to parse, so a broken
de.json left store.Available with only "en" and the parity loop compared
English against itself, always green. Add two guards before the loop:
loaded-locale count must match the number of *.json files in the folder,
and "de" must be present via TryGet, both with failure messages naming the
missing file. Also independently JsonDocument.Parse every *.json so a
syntax error surfaces as a syntax error with the file name, not as
"locale missing". LocaleStore's catch-and-skip is untouched -- a broken
translation must still not crash the app at runtime.

tests/ClaudeDo.Localization.Tests/EnJsonCoverageTests.cs

@@ -1,3 +1,4 @@
+using System.Text.Json;
 using ClaudeDo.Localization;
 
 namespace ClaudeDo.Localization.Tests;
@@ -16,10 +17,34 @@ public class EnJsonCoverageTests
         throw new DirectoryNotFoundException("Could not locate src/ClaudeDo.Localization/locales");
     }
 
+    [Fact]
+    public void All_locale_files_parse_as_valid_json()
+    {
+        var localesDir = LocalesDir();
+        foreach (var path in Directory.EnumerateFiles(localesDir, "*.json"))
+        {
+            try
+            {
+                JsonDocument.Parse(File.ReadAllText(path));
+            }
+            catch (JsonException ex)
+            {
+                Assert.Fail($"{Path.GetFileName(path)} is not valid JSON: {ex.Message}");
+            }
+        }
+    }
+
     [Fact]
     public void Every_locale_has_same_keys_as_english()
     {
-        var store = LocaleStore.Load(LocalesDir());
+        var localesDir = LocalesDir();
+        var jsonFileCount = Directory.EnumerateFiles(localesDir, "*.json").Count();
+        var store = LocaleStore.Load(localesDir);
+        Assert.True(store.Available.Count == jsonFileCount,
+            $"Expected {jsonFileCount} locale(s) to load from {localesDir}, but only {store.Available.Count} loaded " +
+            $"(loaded: {string.Join(", ", store.Available.Select(l => l.Code))}). A *.json file failed to parse.");
+        Assert.True(store.TryGet("de", out _), "Locale 'de' did not load — de.json failed to parse.");
+
         Assert.True(store.TryGet("en", out var en));
         var enKeys = en!.Strings.Keys.ToHashSet();