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

docs/IMPLEMENTATION-PLAN.md

@@ -324,6 +324,46 @@ The convergent rules and citations live in that doc; the spec (§06) was amended
 
 ---
 
+## M4.5 · Snippet-library consolidation ✅ (done)
+
+**Why out of band:** a spec-vs-implementation audit after M4 found §02 features that no
+later milestone owned — the **Selected-Snippet Metadata Panel** (inline name + comment
+editing, timestamps, linked datasets) and the **Duplicate** operation. Without them a
+snippet could only ever carry its auto-generated date-time name (no rename, no annotation,
+no copy), a sharp edge for a _snippet manager_. Closed before M5 since the spec text
+already existed and the work was core-first and cheap.
+
+**Core**
+
+- `snippet.ts` — `duplicateSnippet(source, {now,id})`: independent copy carrying both spec
+  versions, comment, tags, and dataset refs; "(copy)" name; fresh identity/timestamps;
+  cloned mutable members.
+
+**App**
+
+- `SnippetStore` — `renameSnippet`, `setComment` (both advance `modified` per §02 → Sort,
+  no editor-buffer touch), `duplicateActiveSnippet` (flushes the live buffer first, prepends
+  the copy, makes it active).
+- `SnippetLibrary` — the metadata panel below the list: Name + Comment auto-save (debounced
+  while typing, flushed on blur), read-only Created/Modified, Linked Datasets list, and
+  Duplicate / Delete. Duplicate raises a success toast (the copy isn't self-evident, unlike
+  Create); §02-compliant.
+
+**Also fixed (§03C divergence):** the preview debounced _every_ change, so a snippet
+load / Draft↔Published switch incurred a 300 ms blank instead of the spec's **immediate**
+render. `LivePreview` now renders immediately on `bufferEpoch`/`editorView` change and
+debounces only keystroke (`shownText`-only) changes.
+
+**Still deferred to M5/M6 (per §02):** Search, Sort controls + persistence, two distinct
+empty-state messages, Storage Monitor.
+
+**Verified:** `typecheck` + `test` (287 passing — `snippet` duplicate factory,
+`SnippetStore` rename/comment/duplicate, a `SnippetLibrary` render test guarding the
+auto-save effect against a render loop) + `eslint` clean + `build` (PWA, 41 precache
+entries).
+
+---
+
 ## M5 · Settings + Import/Export
 
 **Goal:** preferences and whole-workspace backup/transfer.