Getting the existing symbols could be easier

Right now we iterate through all the pages and then all the layers to get the symbol masters.

There is a convenient method on `MSDocumentData` called `allSymbols` and we should just use it.

Locate the code that iterates through pages and layers to collect symbol masters, then inspect the MSDocumentData entry point and its allSymbols method. Done means the existing symbol lookup uses allSymbols instead of nested page and layer iteration; verify the affected behavior with the repository's available checks.

Written by the indexing model from the issue text.