[2.0] Writing to a store outside its setter is silently ignored #4179

Describe the bug

Writing to a store proxy outside setStore does nothing: no write, no error, no dev diagnostic, and no type error.

• The proxy's set trap returns true without writing outside a draft (packages/signals/src/store/store.ts, if (!draft && !override) return true;). deleteProperty behaves the same way.
• Store<T> = T (store/types.ts), so TypeScript doesn't flag it either.

Your Example Website or App

Minimal repro inline below (signals-only).

Steps to Reproduce the Bug or Issue

import { createStore, createRoot, createRenderEffect, flush } from "@solidjs/signals";

const [store, setStore] = createStore({ n: 0, items: [] as number[], user: { name: "a" } });
createRoot(() => createRenderEffect(() => store.n, v => console.log("n", v)));
flush();

store.n = 5;
store.items.push(1);
store.user.name = "b";
delete (store as any).user;
flush();
// store.n === 0, store.items.length === 0, store.user.name === "a"
// Nothing throws, nothing is logged, the effect doesn't run.

setStore(s => { s.n = 5; }); flush(); // works: logs "n 5"

Expected behavior

In dev, a direct write should be reported. For example, a STORE_WRITE_OUTSIDE_SETTER diagnostic, emitted once per store path, with the path and a setStore(s => …) hint. Production behavior would stay the same.

Optionally, an opt-in deep-readonly Store<T> type. That's a bigger change, because it affects passing store data to functions that take mutable types.

Screenshots or Videos

No response

Platform

• OS: macOS (Darwin 25.5)
• Browser: n/a (reproduced in Node v24.18.0 with vitest, importing source)
• Version: solid next @ b0ad1df (2026-10-20, after 2.0.0-rc.16)

Additional context

Mutating state directly is the default habit for people coming from MobX or Vue, and for coding agents writing Solid. Today the result is a UI that silently doesn't update. Most other "this does nothing" cases in 2.0 have a diagnostic code. This came up while writing and verifying the From React docs additions (solidjs/solid-docs#1664).

— Claude, on behalf of @devagrawal09