Keep model compilation headless (#49)

## Summary

- always wrap NWN model compilation with `xvfb-run`, even when the caller has `DISPLAY`
- fail closed with an actionable error when `xvfb-run` is unavailable instead of opening the client UI
- update the compiler contract and regression coverage

## Verification

- focused red/green regression tests
- `nix develop -c make check`
- real NWN compile with `DISPLAY=:0` under transient Xvfb; binary MDL output verified

Generated with Claude CodeReviewed-on: #49

Co-authored-by: vickydotbat <vickydotbat@tutamail.com>

docs/superpowers/specs/2026-07-12-crucible-assets-builder-design.md

@@ -126,8 +126,9 @@ exactly as the reference does:
   - Beamdog install dirs.
   - `--nwn <install>` overrides (path to the install root or directly to the
     binary).
-- **Headless.** The engine is a GUI binary. If there is no `DISPLAY` and
-  `xvfb-run` is present, wrap the call:
+- **Headless.** The engine is a GUI binary. Require `xvfb-run` and wrap the call
+  regardless of the caller's `DISPLAY` so compilation never opens the client
+  UI; fail before invoking the engine when `xvfb-run` is unavailable:
   `xvfb-run -a --server-args=-screen 0 1024x768x24 nwmain-linux compilemodel <stem>`.
 - **Per model (one at a time — the engine's `development/` and `modelcompiler/`
   folders are flat and single-slot):**

internal/assets/compile.go

@@ -40,13 +40,13 @@ func runCompile(args []string, stdout, stderr io.Writer, getenv func(string) str
 	}
 	binDir := filepath.Dir(nwmain)
 
-	// Headless wrap: no DISPLAY + xvfb-run present -> run under a virtual X.
-	var wrap []string
-	if getenv("DISPLAY") == "" {
-		if xvfb := look("xvfb-run"); xvfb != "" {
-			wrap = []string{xvfb, "-a", "--server-args=-screen 0 1024x768x24"}
-		}
+	// Always use a virtual X so compilation never opens the client UI.
+	xvfb := look("xvfb-run")
+	if xvfb == "" {
+		fmt.Fprintln(stderr, "assets compile: xvfb-run not found — install it to run the NWN model compiler headlessly")
+		return exitTool
 	}
+	wrap := []string{xvfb, "-a", "--server-args=-screen 0 1024x768x24"}
 
 	files, err := walk(dirs, mdlExt, !*nonRecursive)
 	if err != nil {

internal/assets/compile_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 )
 
@@ -23,13 +24,19 @@ func TestCompileDrivesEngineAndReplacesInPlace(t *testing.T) {
 	if err := os.WriteFile(nwmain, []byte("#!/bin/sh\n"), 0o755); err != nil {
 		t.Fatal(err)
 	}
+	xvfbDir := t.TempDir()
+	xvfb := filepath.Join(xvfbDir, "xvfb-run")
+	if err := os.WriteFile(xvfb, []byte("#!/bin/sh\n"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("PATH", xvfbDir+string(os.PathListSeparator)+os.Getenv("PATH"))
 
 	getenv := func(k string) string {
 		switch k {
 		case "HOME":
 			return home
 		case "DISPLAY":
-			return ":0" // pretend a display exists so no xvfb wrap is needed
+			return ":0" // a desktop display must not make compilation interactive
 		}
 		return ""
 	}
@@ -38,7 +45,11 @@ func TestCompileDrivesEngineAndReplacesInPlace(t *testing.T) {
 	orig := runner
 	defer func() { runner = orig }()
 	runner = func(dir string, env []string, name string, args ...string) ([]byte, error) {
-		// args: compilemodel <stem>
+		if name != xvfb || len(args) != 5 || args[0] != "-a" ||
+			args[1] != "--server-args=-screen 0 1024x768x24" || args[2] != nwmain ||
+			args[3] != "compilemodel" {
+			t.Fatalf("engine command = %q %q, want xvfb-run wrapping nwmain", name, args)
+		}
 		stem := args[len(args)-1]
 		compiled := filepath.Join(mc, stem+".mdl")
 		return nil, os.WriteFile(compiled, []byte("\x00\x00compiled"), 0o644)
@@ -77,6 +88,11 @@ func TestCompileAbortsOnNameMismatch(t *testing.T) {
 	if err := os.WriteFile(nwmain, []byte("#!/bin/sh\n"), 0o755); err != nil {
 		t.Fatal(err)
 	}
+	xvfbDir := t.TempDir()
+	if err := os.WriteFile(filepath.Join(xvfbDir, "xvfb-run"), []byte("#!/bin/sh\n"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("PATH", xvfbDir+string(os.PathListSeparator)+os.Getenv("PATH"))
 	getenv := func(k string) string {
 		if k == "HOME" {
 			return home
@@ -108,3 +124,48 @@ func TestCompileAbortsOnNameMismatch(t *testing.T) {
 		t.Fatal("engine should not run for a name-mismatched model")
 	}
 }
+
+func TestCompileFailsClosedWithoutXvfb(t *testing.T) {
+	home := t.TempDir()
+	userData := filepath.Join(home, ".local", "share", "Neverwinter Nights")
+	for _, d := range []string{filepath.Join(userData, "development"), filepath.Join(userData, "modelcompiler")} {
+		if err := os.MkdirAll(d, 0o755); err != nil {
+			t.Fatal(err)
+		}
+	}
+	nwmain := filepath.Join(t.TempDir(), "nwmain-linux")
+	if err := os.WriteFile(nwmain, []byte("#!/bin/sh\n"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("PATH", t.TempDir())
+
+	getenv := func(k string) string {
+		if k == "HOME" {
+			return home
+		}
+		return ""
+	}
+	orig := runner
+	defer func() { runner = orig }()
+	engineCalled := false
+	runner = func(string, []string, string, ...string) ([]byte, error) {
+		engineCalled = true
+		return nil, nil
+	}
+
+	srcDir := t.TempDir()
+	if err := os.WriteFile(filepath.Join(srcDir, "foo.mdl"),
+		[]byte("newmodel foo\nbeginmodelgeom foo\n  node dummy foo\n    parent null\n  endnode\nendmodelgeom foo\ndonemodel foo\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	var stdout, stderr bytes.Buffer
+	if code := runCompile([]string{"--nwn", nwmain, srcDir}, &stdout, &stderr, getenv); code != exitTool {
+		t.Fatalf("compile exit = %d, want %d\n%s", code, exitTool, stderr.String())
+	}
+	if engineCalled {
+		t.Fatal("engine must not run without xvfb-run")
+	}
+	if !strings.Contains(stderr.String(), "xvfb-run") {
+		t.Fatalf("missing actionable xvfb-run error: %s", stderr.String())
+	}
+}