Add a translation assistant to the panel

Each field's original sits beside the field to write, and a translation that
fell behind shows what the original said when it was translated and what it
says now, word by word.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/Support/TranslationFreshness.php

@@ -71,6 +71,69 @@ final class TranslationFreshness
     /**
      * @return array<string, CarbonInterface>
      */
+    /**
+     * Field by field, for one language: which fields the original changed
+     * after this language was last written, since when, and what the
+     * original said back then — so a translator can see what to redo rather
+     * than rereading the whole text.
+     *
+     * @return array<string, array{since: CarbonInterface, was: ?string}>
+     */
+    public static function staleFields(Model $record, string $locale): array
+    {
+        $source = (string) config('locales.default');
+        $fields = self::fields($record);
+        $touched = self::lastTouched($record, $fields);
+        $stale = [];
+
+        foreach ($fields as $field) {
+            if ($locale === $source || blank($record->getTranslation($field, $locale, false))) {
+                continue;
+            }
+
+            $sourceAt = $touched[$field][$source] ?? null;
+            $translatedAt = $touched[$field][$locale] ?? $record->created_at;
+
+            if ($sourceAt !== null && $translatedAt !== null && $sourceAt->gt($translatedAt)) {
+                $stale[$field] = [
+                    'since' => $sourceAt,
+                    'was' => self::sourceAsOf($record, $field, $translatedAt),
+                ];
+            }
+        }
+
+        return $stale;
+    }
+
+    /**
+     * The original's text in a field as it stood at a moment, from the
+     * history: the last value written at or before it, or the value the first
+     * later change replaced.
+     */
+    protected static function sourceAsOf(Model $record, string $field, CarbonInterface $at): ?string
+    {
+        $source = (string) config('locales.default');
+        $value = null;
+        $found = false;
+
+        foreach ($record->revisions()->reorder()->orderBy('id')->get() as $revision) {
+            $change = $revision->changeset[$field] ?? null;
+
+            if (! is_array($change)) {
+                continue;
+            }
+
+            if ($revision->created_at->lte($at)) {
+                $value = ((array) ($change['new'] ?? []))[$source] ?? null;
+                $found = true;
+            } elseif (! $found) {
+                return ((array) ($change['old'] ?? []))[$source] ?? null;
+            }
+        }
+
+        return $found ? $value : $record->getTranslation($field, $source, false);
+    }
+
     protected static function compute(Model $record): array
     {
         $source = (string) config('locales.default');