[M5+] realize candidate output before nix-cmake-scan

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: `nix_cmake_scan` would silently fail when the dev output had
+  not yet been realized on the local machine — `nixpkgs_probe`'s
+  `nix eval` only *computes* a store path, so packages first-time
+  encountered (`libclang`, anything not previously built) returned a
+  path that didn't exist on disk. Discovery now realizes the
+  candidate output via
+  `nix build --no-link --print-out-paths nixpkgs#<attr>.dev` (or
+  `.<empty>`) before scanning. New free function
+  `cargoxx::resolver::realize_path(flake_attr)` wraps the call.
+  Live verified: `cargoxx add libclang` now reaches the verify-link
+  step (the subsequent `find_dependency(LLVM)` failure inside Clang's
+  CMake config is the pre-existing cross-package transitive-dep
+  limit — workaround is to `cargoxx add libllvm` first).
 - Fix: `nix_cmake_scan` walked only the package's default output. For
   multi-output Nix packages — `boost`, `openssl`, `libllvm`,
   `libclang`, glib, … — CMake configs live in the `dev` output