[M2] add SQLite overlay + add_manual

CHANGELOG.md

@@ -57,3 +57,16 @@ All notable changes to cargoxx will be documented in this file.
   are deferred to the M2 follow-up commit.
   `tests/linkdb_lookup.cpp` covers 13 cases including a smoke test that
   resolves all 25 curated packages.
+- SQLite overlay: `Database::open(overlay_path)` now opens (and creates,
+  if missing) a per-user `linkdb.sqlite` cache, applying the schema from
+  `SPEC.md` §9 idempotently. Default path is
+  `$XDG_CACHE_HOME/cargoxx/linkdb.sqlite` (falling back to
+  `$HOME/.cache/cargoxx/...`); tests pass an explicit temp path so they
+  never touch the user cache. `Database::add_manual(pkg, range, recipe)`
+  inserts a row with `source = 'manual'` and `verified_at = now()`;
+  `resolve()` consults the overlay first and falls back to curated when
+  no overlay row matches the requested version range. Manual entries
+  never expire; `nix-probe` (v0.2) entries respect a 30-day freshness
+  window. `tests/linkdb_overlay.cpp` covers 7 cases (insert/persist,
+  override-curated, version-range gating, components rejection,
+  move semantics).