Add review guardrails for changes to Salsa declarations

[lionel-]

Part of #1409

This PR adds review guardrails when the Salsa declaration surface changes: a new query, different key, or changed recovery policy now leaves an explicit snapshot diff that asks the reviewer to reconsider cycle safety. The reviewer is pointed to a new salsa.md guide.

The snapshots are generated by a new oak_tidy crate. The name "tidy" follows other projects where "tidying" means checking source-level properties that the type system can't enforce:

• https://github.com/rust-lang/rust-analyzer/blob/master/xtask/src/tidy.rs
• https://github.com/tigerbeetle/tigerbeetle/blob/main/src/tidy.zig

Oak's Rust source is walked with the syn crate to find Salsa queries and cycle handlers. The collected data is snapshotted with insta.

Note that the snapshot is necessarily partial: it only captures the Salsa declarations, not simple functions calling these queries. It doesn't capture Salsa edges in general because they can arise from any kind of Rust code invocation (methods, closures, functions), and can't be traced in the general case.

In addition to its role in prompting agents for a review when it changes, the snapshot is also a useful reference for both us and agents.