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 === "legacy");
+      definitions.find((entry) => entry.runtime === "shared");
     if (!selected) continue;
     Reflect.set(actions, name, async (...args: unknown[]) => {
       currentAction = name;