[M5+] add resolver::nixpkgs_probe (nix eval wrapper)

CHANGELOG.md

@@ -78,6 +78,17 @@ All notable changes to cargoxx will be documented in this file.
   and `[build]` honoring `warnings_as_errors` and `sanitizers`. Source
   paths emitted relative to `build/` (i.e. prefixed with `../`).
   Output is deterministic. `tests/codegen_cmake.cpp` covers 11 cases.
+- `cargoxx.resolver::nixpkgs_probe(attr)` — runs
+  `nix eval nixpkgs#<attr> --json --apply 'p: { version, path }'` via
+  `exec::run` and returns a `NixpkgsInfo { attr, version, out_path }`.
+  Distinguishes missing attributes (`ResolutionUnknownPackage`) from
+  evaluator/network errors (`ResolutionNetworkError`). The `path` field
+  name avoids nix's `outPath`-driven derivation coercion in `--json`
+  mode (which would otherwise serialize the result as a bare path
+  string instead of a JSON object). Pure parser
+  `parse_nix_eval_json(attr, text)` exposed for unit tests; live tests
+  in `tests/nixpkgs_probe_live.cpp` are gated behind
+  `CARGOXX_NETWORK_TESTS=1` (verified locally against `hello`).
 - `cargoxx add <pkg>[@<version>] [--components a,b]` edits `Cargoxx.toml`,
   validates the new dep against the curated linkdb (so unknown packages
   and missing components fail before any disk write), and rejects