fix(persist): delete IndexedDB keys for symbolDelete

[Steve0x2a]

When a persist change uses Legend-State's existing symbolDelete sentinel, the IndexedDB plugin treated it as a stored value and called IDBObjectStore.put(). IndexedDB uses the structured clone algorithm, which cannot clone Symbol, so every persisted deletion crashed:

DataCloneError: Failed to execute 'put' on 'IDBObjectStore': Symbol(delete) could not be cloned.

_setItem already deletes on falsy values. symbolDelete is truthy, so it fell through to put. This also hits the full-table path because _setTable delegates each entry to _setItem.

The plugin now treats symbolDelete like the existing delete path and calls store.delete(key) instead of putting the Symbol. No new delete protocol.

Credits @LinusOssmann for the report and repro.

Fixes #658