Add snippet metadata panel, duplicate, and immediate-load preview (M4.5)

src/core/snippet.ts

@@ -126,6 +126,35 @@ export function createSnippet(options: CreateSnippetOptions = {}): Snippet {
   };
 }
 
+export interface DuplicateSnippetOptions {
+  /** Clock injection for deterministic tests; defaults to the current time. */
+  now?: Date;
+  /** Id injection for deterministic tests; defaults to a random UUID. */
+  id?: string;
+}
+
+/**
+ * Create an independent copy of a snippet (spec §02 → Duplicate). The copy carries
+ * over the specification (both published and draft), comment, tags, and dataset
+ * references, gets a new identity and fresh created/modified timestamps, and a name
+ * suffixed "(copy)". Mutable members are cloned so the copy shares no references
+ * with its source.
+ */
+export function duplicateSnippet(source: Snippet, options: DuplicateSnippetOptions = {}): Snippet {
+  const iso = (options.now ?? new Date()).toISOString();
+  return {
+    ...source,
+    id: options.id ?? crypto.randomUUID(),
+    version: CURRENT_SNIPPET_VERSION,
+    name: `${source.name} (copy)`,
+    created: iso,
+    modified: iso,
+    tags: [...source.tags],
+    datasetRefs: [...source.datasetRefs],
+    meta: { ...source.meta },
+  };
+}
+
 /** True when the snippet's draft differs from its published spec (§03D). */
 export function hasUnpublishedChanges(snippet: Snippet): boolean {
   return snippet.draftSpec !== snippet.spec;