docs(core): document hashbrown feature's fixed-key hashing

docs(core): document hashbrown feature's fixed-key hashing

With the `hashbrown` feature, this crate's `HashMap`/`HashSet` are
hashbrown's (on `std` builds too), which hash with `ahash`. Unless
`ahash`'s `runtime-rng` or `compile-time-rng` feature is enabled
somewhere in the dependency graph, `ahash` uses fixed, publicly known
keys, so a malicious chain source can craft colliding txids and make
`TxGraph` insertion quadratic.

Document this, and how to randomize the keys by depending on `ahash`
directly with one of those features.

Fixes #2342

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>