feat(builder): pinpoint settings a server can't start without

Booting drafts as deployed catches a server that stops when a setting is
empty, but the server's own output rarely says why, so the model guessed
(more memory) for three rounds and the draft ended as WIP.

When a draft fails with its defaults and has blank password-like settings
without x-generate, the builder boots it once more with those filled. If
that boot passes, the cause is certain, and the model is told exactly that:
these settings stop the server when empty; give them x-generate (a password
per server) or a working default.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/gbrgames/build_test.go

@@ -1,6 +1,8 @@
 package main
 
 import (
+	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 )
@@ -48,3 +50,30 @@ func TestVariantNotes(t *testing.T) {
 		}
 	}
 }
+
+func TestBlankRequiredLookingSettings(t *testing.T) {
+	dir := t.TempDir()
+	game := filepath.Join(dir, "example")
+	os.MkdirAll(game, 0o755)
+	os.WriteFile(filepath.Join(game, "game.yaml"), []byte(`id: example
+defaultConfig:
+  JoinPassword: ""
+  AdminPassword: ""
+  ServerPassword: set-by-default
+  ServerName: x
+configSchema:
+  properties:
+    JoinPassword: {type: string}
+    AdminPassword: {type: string, x-generate: {type: password}}
+    ServerPassword: {type: string}
+    ServerName: {type: string}
+`), 0o644)
+	got := blankRequiredLooking(dir, "example")
+	if len(got) != 1 || got[0] != "JoinPassword" {
+		t.Errorf("got %v, want only JoinPassword (AdminPassword is generated, ServerPassword has a default)", got)
+	}
+	msg := emptySettingsProblem(got, "FAILED: server crashed")
+	if !strings.Contains(msg, "stops when JoinPassword is empty") || !strings.Contains(msg, `"x-generate": {"type": "password"}`) {
+		t.Errorf("message: %s", msg)
+	}
+}