ci: macOS smoke-test — run tests + nix-store generation pruning

- Add `cargo test` before the darwin builds (gate is Linux-only; the macOS
  leg is test-then-build) — a full dry-run of release-macos bar the upload.
- Add an `if: always()` prune step. The runner wipes the workspace each run,
  so cargo target/ never accumulates (no sweep). The persistent cache is the
  nix store: record the current toolchain in a persistent profile, keep the
  2 newest generations (nix-env --delete-generations +2), reclaim the rest
  (nix-collect-garbage). Pairs with min-free/max-free in the runner nix.conf.

.gitea/workflows/macos-probe.yaml

@@ -25,6 +25,8 @@ jobs:
       NIX_CONFIG: "experimental-features = nix-command flakes"
     steps:
       - uses: actions/checkout@v4
+      - name: test (macOS — the gate only covers Linux)
+        run: nix develop -c cargo test --no-fail-fast
       - name: build, de-nix, sign, verify both darwin targets
         run: |
           set -e
@@ -58,3 +60,22 @@ jobs:
             echo "OK: $t portable"
           done
           echo "=== both darwin targets built, de-nixed, signed, verified ==="
+
+      - name: prune nix store — keep the last 2 toolchain generations
+        # The runner wipes the whole workspace before each run, so cargo target/
+        # never accumulates (no sweep needed). The persistent caches are the nix
+        # store (/nix) and ~/.cargo (in $HOME). Bound the nix store by generation:
+        # record the current devShell closure as a generation of a persistent
+        # profile (lives in $HOME, survives the workspace wipe), keep the 2 newest
+        # (current + previous), reclaim what the older ones referenced. No time
+        # window — never more than two toolchains regardless of flake.lock churn.
+        if: always()
+        run: |
+          echo "--- disk before ---"; df -h / | tail -1
+          P="$HOME/.cache/rdbms-ci/toolchain"
+          nix develop --profile "$P" -c true || true
+          nix-env -p "$P" --delete-generations +2 || true
+          nix-collect-garbage || true
+          echo "--- disk after ---"; df -h / | tail -1
+          # ~/.cargo/registry also persists but grows only on Cargo.lock bumps;
+          # bound it later with `cargo-cache --autoclean` if it ever matters.