[M5+] add resolver::nixpkgs_git_resolve fallback

CHANGELOG.md

@@ -91,6 +91,18 @@ All notable changes to cargoxx will be documented in this file.
   6 cases against fixtures derived from a real fmt 10.2.1 response;
   `tests/devbox_resolve_live.cpp` (gated by `CARGOXX_NETWORK_TESTS=1`)
   hits the live API.
+- `cargoxx.resolver::nixpkgs_git_resolve(name, version, repo_path?)` —
+  fallback for when search.devbox.sh is unreachable. Lazily clones
+  `https://github.com/NixOS/nixpkgs.git` into
+  `$XDG_CACHE_HOME/cargoxx/nixpkgs/` (or `$HOME/.cache/...`) on first
+  use, then runs
+  `git -C <repo> log --all -S 'version = "<v>"' --pretty='%H %ct' -- pkgs/`
+  and returns the youngest matching commit. Pure helper
+  `pick_youngest_commit(text)` parses the `%H %ct` lines. The unit
+  test builds a tiny throwaway git fixture that mirrors the
+  `pkgs/development/libraries/<pkg>/default.nix` layout (avoiding the
+  real multi-GB clone) and verifies introducing-commit detection plus
+  the not-found path; 5 cases.
 - `cargoxx add <pkg>` now auto-resolves packages outside the curated
   linkdb. On `LinkdbUnknownPackage`, `cmd_add` invokes
   `resolver::discover` which: probes `nixpkgs#<pkg>` to confirm the