fix(store): drop unreachable legacy StoreRuntime fallback

Fix round 2 for task 1: store-codegen.ts stopped emitting runtime:
"legacy" actions in round 1, making the StoreRuntime variant and its
resolution fallback dead. Narrows StoreRuntime to three variants and
adds a regression test for the remaining options.runtime -> shared
fallback chain.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/store/src/index.ts

@@ -382,8 +382,7 @@ export function createStoreInstance<
     const definitions = Array.isArray(rawDefinitions) ? rawDefinitions : [rawDefinitions];
     const selected =
       definitions.find((entry) => entry.runtime === options.runtime) ??
-      definitions.find((entry) => entry.runtime === "shared") ??
+      definitions.find((entry) => entry.runtime === "shared");
-      definitions.find((entry) => entry.runtime === "legacy");
     if (!selected) continue;
     Reflect.set(actions, name, async (...args: unknown[]) => {
       currentAction = name;

packages/store/src/types.ts

@@ -1,5 +1,5 @@
 export type StoreKind = "global" | "page";
-export type StoreRuntime = "shared" | "client" | "server" | "legacy";
+export type StoreRuntime = "shared" | "client" | "server";
 export type PersistenceStorage = "memory" | "session" | "local";
 /** Broad callable constraint that preserves each action's concrete parameters and return type. */
 export type StoreFunction = (...args: any[]) => unknown;

packages/store/test/legacy-runtime-removal.test.ts

@@ -0,0 +1,41 @@
+import { expect, test } from "bun:test";
+import { createStoreContainer, defineStore } from "../src/index.ts";
+
+// Regression guard for the removed `"legacy"` StoreRuntime fallback.
+//
+// `createStoreInstance` resolves an action definition by walking:
+//   exact `options.runtime` match -> "shared" match -> (previously) "legacy" match
+// The `"legacy"` step was dead (nothing ever produced a `runtime: "legacy"`
+// action once `store-codegen.ts` stopped emitting it) and has been removed.
+// This test locks in the remaining `options.runtime` -> `"shared"` chain so a
+// future edit can't silently drop the `"shared"` fallback too.
+const OnlySharedActionStore = defineStore({
+  name: "OnlySharedActionStore",
+  kind: "global" as const,
+  createSharedState: () => ({ count: 0 }),
+  actions: {
+    // No "client" or "server" definition exists for this action — only
+    // "shared". A server-runtime container must still resolve it via the
+    // "shared" fallback.
+    increment: {
+      runtime: "shared" as const,
+      handler: ({ state }: any, amount = 1) => {
+        state.count += amount;
+      },
+    },
+  },
+});
+
+test("an action with only a shared definition resolves via the shared fallback on the server runtime", async () => {
+  const container = createStoreContainer({ runtime: "server", request: {} });
+  const store = await container.use(OnlySharedActionStore);
+  await store.increment(5);
+  expect(store.count).toBe(5);
+});
+
+test("an action with only a shared definition resolves via the shared fallback on the client runtime", async () => {
+  const container = createStoreContainer({ runtime: "client" });
+  const store = await container.use(OnlySharedActionStore);
+  await store.increment(3);
+  expect(store.count).toBe(3);
+});