fix(test): store tests take an OS-assigned free port, not a PID-derived one

PID%1000 collided with a concurrent job on the shared CI host (run on
e946709: process already listening on port 54000). freePort asks the OS;
Start is retried up to 3 times if the port is taken in between.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SZkhi49hfeSqBp5aZJNXsM

internal/adapters/store/port_test.go

@@ -0,0 +1,19 @@
+package store_test
+
+import (
+	"fmt"
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// TestMain's Postgres already holds a port. A second instance started in the
+// same process must still come up, i.e. the port is not derived from something
+// two starters share (the PID). Observes: startTestPostgres succeeds while
+// another instance from this process is listening.
+func TestStartTestPostgres_SecondInstanceStartsWhileFirstHoldsItsPort(t *testing.T) {
+	pg, _, rt, err := startTestPostgres(fmt.Sprintf("tapir-epg-store-second-%d", os.Getpid()))
+	require.NoError(t, err)
+	t.Cleanup(func() { _ = pg.Stop(); _ = os.RemoveAll(rt) })
+}