Pin gix to nikicat/gitoxide#b-plus-c-integration for AUR-mirror perf wins

Combines two upstream PRs that are not yet released:
  - GitoxideLabs/gitoxide#2604 — gix-ref: skip name validation in
    packed-refs binary search.
  - GitoxideLabs/gitoxide#2605 — gix: cache packed-refs in a HashMap
    during fetch update_refs.

On `gitaur -Sy` against the AUR mirror (154k branches in packed-refs,
warm cache, no incoming updates) the combo brings wall time from
~11.0s on stock gix 0.83 to ~3.4-5.0s, putting gitaur on par with
system `git fetch` on the same workload. Profile evidence:
docs/PROFILING.md plus the `gitaur-bc.json.gz` samply capture.

Revert to a versioned crates.io dependency once both PRs land and a
gix release ships with them.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: nikicat