Add doc mocks and a tiny (starter) integration test suite

[j6k4m8]

Adds new mock/debugging objects:

• InMemoryPaperRemote: stores paper elements as list[str] in memory
• WriteOutDigitsEditorAgent: a basic editor agent that just converts digits like "1" to words like "one", as a networkless, AIless debug tool

Adds new type hierarchy to PaperRemotes:

• OverleafGitPaperRemote now inherits from MultiDocumentPaperRemote
• SingleDocumentPaperRemote spec now explicitly states that document IDs are ignored. This is kinda the ugliest thing in this codebase right now, but makes it possible to not have to rewrite all of our types to be document-ID-agnostic

Adds new convenience types:

• DocumentID is just a Hashable, but because it's type-aliased, it gives us a nice type annotation to keep straight what we're representing in function signatures et al

Adds test suite...

...the idea of which right now is to just make it slightly harder to accidentally break existing APIs without getting a big ❌ on GitHub PRs, and which doesn't really accomplish that much in the way of unittesting at all

• Basic integration tests for PaperRemote
• Basicker integration tests for EditorAgent protocol

[j6k4m8]

@wrongu take a look and let me know if this is ready to merge?

~I also have some thoughts on the editresult types, i'll add them in #21~ [edit] you already thought of them too :)