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>

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 {