[M5+] use devbox attr_paths for pinned-dep nixpkgs attr

CHANGELOG.md

@@ -91,6 +91,19 @@ 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.
+- Fix: pinned-dep `buildInputs` line used the curated linkdb's
+  `nixpkgs_attr` (e.g. `fmt_10`) regardless of the pinned rev, so
+  `cargoxx add fmt@12.1.0` emitted `pkgs_nixpkgs_fmt_12_1_0.fmt_10`
+  even though that rev only exposes `fmt`. `resolve_version` now
+  returns `ResolvedVersion { nixpkgs_rev, nixpkgs_attr }` (the attr
+  comes from devbox's `attr_paths` for the resolved rev, with the
+  package name as a best-effort fallback for the git path).
+  `cmd_add` writes the attr into the lockfile and `cmd_build`'s
+  `merge_lockfile` now also preserves it. Codegen prefers the
+  lockfile's attr over the linkdb recipe's whenever it's set, so
+  pinned deps reach the right attribute on their per-dep nixpkgs.
+  Live verified: `cargoxx add fmt@12.1.0 && cargoxx build` now emits
+  `pkgs_nixpkgs_fmt_12_1_0.fmt` and the binary builds and runs.
 - `flake.nix` codegen rewritten for **per-dep nixpkgs revisions**. The
   shared `nixpkgs` input always tracks `nixos-unstable` and provides
   the toolchain. Each pinned manifest dep