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):**