feat(model): add IndexedDB-backed model repository

src/model/repository.ts

@@ -0,0 +1,48 @@
+/**
+ * Model repository (C9, F7) — where the working document is persisted locally.
+ *
+ * IndexedDB (via `idb`) sits behind a tiny `ModelRepository` interface so the
+ * storage engine is no longer a hard-to-reverse choice (T4): a future sync
+ * backend (PouchDB, an API) is a new implementation behind the same two methods,
+ * with no caller change. Phase 1 is single-document, so the current Model is
+ * stored under one fixed key; a model list (C12) would key by `model.id` instead.
+ */
+import { type IDBPDatabase, openDB } from "idb"
+import type { Model } from "./types"
+
+export interface ModelRepository {
+  /** Persist the current Model, overwriting the previous autosave. */
+  save(model: Model): Promise<void>
+  /** Load the autosaved Model, or null if nothing has been saved yet. */
+  load(): Promise<Model | null>
+}
+
+const DB_NAME = "meadows"
+const DB_VERSION = 1
+const STORE = "models"
+const CURRENT_KEY = "current"
+
+let dbPromise: Promise<IDBPDatabase> | null = null
+
+function db(): Promise<IDBPDatabase> {
+  // Memoise the open so we pay the handshake once, not per save.
+  dbPromise ??= openDB(DB_NAME, DB_VERSION, {
+    upgrade(database) {
+      if (!database.objectStoreNames.contains(STORE)) database.createObjectStore(STORE)
+    },
+  })
+  return dbPromise
+}
+
+/** The default IndexedDB-backed repository used by the app. */
+export function createRepository(): ModelRepository {
+  return {
+    async save(model: Model): Promise<void> {
+      await (await db()).put(STORE, model, CURRENT_KEY)
+    },
+    async load(): Promise<Model | null> {
+      const model = await (await db()).get(STORE, CURRENT_KEY)
+      return (model as Model | undefined) ?? null
+    },
+  }
+}