[M5+] wire resolver::discover into cargoxx add

CHANGELOG.md

@@ -78,6 +78,21 @@ 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 add <pkg>` now auto-resolves packages outside the curated
+  linkdb. On `LinkdbUnknownPackage`, `cmd_add` invokes
+  `resolver::discover` which: probes `nixpkgs#<pkg>` to confirm the
+  attribute exists and capture its store path; tries Conan, vcpkg, and
+  the nix-store CMake scan in order; for each candidate runs a real
+  `cargoxx build` against an empty `int main() {}` project via
+  `verify_link`; on first success persists a confirmed overlay row.
+  Subsequent `cargoxx add <pkg>` for the same package is an
+  overlay-cache hit (~ms). End-to-end live: a fresh `cargoxx add
+  simdjson` (not in our curated db) takes ~11 s including the verifying
+  nix+cmake build, then 0.002 s on the second invocation.
+  `linkdb::default_overlay_path()` is now exported as a public helper
+  so the resolver and CLI agree on the overlay file when no path is
+  explicitly passed. Tests opt out of the slow chain via
+  `CARGOXX_NO_AUTORESOLVE=1`.
 - `cargoxx.linkdb::Database::insert_provisional`,
   `confirm_provisional`, and `abort_provisional` — three-step lifecycle
   for non-`manual` overlay rows: insert with `verified_at = 0`, run a