fix(storage): fall back to memory when IndexedDB is unavailable

[erhnysr]

Problem

Storage.idb() guards on window before selecting IndexedDB-backed storage, but never checks for indexedDB itself. In a runtime that exposes a partial window without IndexedDB — React Native and Expo, some SSR setups — idb() is still chosen as the default, and the first read or write throws ReferenceError: indexedDB is not defined.

The underlying cause is that idb-keyval treats an undefined store as "use the default store", which opens IndexedDB unconditionally. So the existing typeof indexedDB !== 'undefined' check inside the function never prevents anything — the delegation happens anyway.

AGENTS.md:106 already calls this out as a rule:

React Native may expose window without IndexedDB — storage defaults that mean "browser" should guard on indexedDB, not window, or Expo/React Native can try to use IndexedDB-backed storage and throw at startup.

This change brings the code into line with that rule rather than introducing a new policy.

Fix

Storage.idb() now falls back to the existing in-memory adapter when IndexedDB isn't available, instead of delegating to idb-keyval's default store.

Putting the fallback inside idb() fixes it at the root: all three default-storage call sites funnel through this function, so a partial-window runtime gets a working adapter — session persistence with structured-clone semantics via memory()'s markStructuredClone — rather than a startup crash. No changes to Provider.ts.

Tests

src/core/Storage.test.ts asserts typeof indexedDB === 'undefined' as a precondition, then exercises set / get / remove through idb(). Verified failing-first: before the change the first setItem throws.

Changeset

patch. No new API and no new capability for setups that already work — a crash becomes functioning degraded behaviour. This matches the bump used for the directly comparable guard fixes, #706 (crypto.randomUUID guard) and #734 (partial-window announcement guard).

Note on the trade-off

An app that previously crashed loudly at startup will now silently persist to memory instead, so persistence across reloads won't happen in that runtime. That's the same way the window-guarded defaults already degrade for SSR, and it's what the existing indexedDB check was evidently meant to do.

If you'd rather fail loudly — a clear Error('Storage.idb requires IndexedDB') instead of degrading — that's a reasonable alternative, but it would leave the React Native default path crashing unless the three call sites also switch to memory(). Happy to take it that direction if you prefer.

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