feat: rehydrate lazy-loaded reducers on demand (store.rehydrate)

[KAMRONBEK]

Adds first-class support for lazy-loaded / injected reducers via a new store.rehydrate(keys?) method. Addresses #26.

In #26 the concern was that injecting a persisted slice races the one-time rehydration: rehydrate() fires once at init, so a reducer injected later misses its stored value and starts from its initial state. This adds an explicit, opt-in way to load it after injection:

rootReducer.inject(lazyPersistedSlice);   // add the reducer
await store.rehydrate(['lazyPersisted']); // load its persisted state + keep persisting it

• store.rehydrate(keys) reads those keys from storage, dispatches REMEMBER_REHYDRATED, and adds them to the remembered set so they persist from then on. With no arguments it re-reads every currently remembered key.
• Non-persisted injected slices already worked (they just aren't in rememberedKeys); that case is documented too.

Changes:

• index.ts — attach store.rehydrate (reuses the existing internal rehydrate); the enhancer returns Object.assign(store, { rehydrate }).
• types.ts — RehydrateFunction + RememberEnhancerStoreExt, set as the enhancer's default Ext so store.rehydrate is typed without extra generics.
• tests — 3 new cases in index.test.ts; full suite 7812 pass, index.ts 100% covered; typecheck + lint clean.
• docs — usage/lazy-loaded-reducers documents both cases and is linked in the sidebar.

Verified end-to-end against a real store + async driver: inject a slice, call store.rehydrate([key]), and it loads the persisted value; without the call it stays at its default (the race described in the issue).

A couple of things I'd defer to you:

• naming (store.rehydrate vs something more scoped), and whether you'd rather it be opt-in behind an option instead of always attached.
• the on-demand path currently reuses your migrate (identity by default). Happy to skip migrate on store.rehydrate if you'd prefer it only run at init.

I know lazy slices weren't on the roadmap, so if you'd rather not add API surface I'm glad to trim this back to just the docs page.