Split rslib into smaller crates to speed up incremental builds

## Problem

The anki crate (rslib/) is a very large crate. Even a small change to any file in rslib/ forces a full recompilation of the entire crate, resulting in very long build times.

## Proposed Solution

Split rslib into smaller, feature-specific sub-crates (e.g. anki_scheduler, anki_search, anki_sync, anki_storage). This way only the sub-crate containing the changed file needs recompiling.

There is already a precedent for this in the form of rslib/i18n, rslib/proto subcrates.

Start by examining the large anki crate in rslib/ and the existing rslib/i18n and rslib/proto subcrates as precedents. Identify feature-specific boundaries for smaller crates such as anki_scheduler, anki_search, anki_sync, and anki_storage; done means changes recompile only the affected sub-crate rather than all of rslib/.

Written by the indexing model from the issue text.