fix(tools): importers clear the read-only LFS bit before saving; docs refresh

0.1.3 packaging and setup fixes from what 0.1.x actually exposed.

- New Tools/ue_common.py: make_writable() clears the read-only bit an LFS .umap
  checks out with (save_map() otherwise returns False silently and the staged actor
  is lost) and require_saved() makes that failure visible. ue_hero_facade_import and
  ue_import_north_buildings now call it; the map builders already did. Verified from
  a forced 0o444 map: chmod logged, save_map True, exit 0.
- Docs/build.md: pipeline table now lists the seven site FBXs plus frontage, north,
  hero and map steps and the shared FBX preset; packaging archive corrected to
  Builds/GigLineQA; packaged-log location and -qaselftest usage documented; new
  'Traps that cost real time in 0.1.x' table (read-only save, hero --out is the FBX,
  MPFB add-on path, shot actor quitting under QA, null-RHI trace trap).
- Docs/setup.md: fresh-checkout build marked verified 2026-11-08 and the blocker
  closed; new optional-keys section for OPENROUTER_API_KEY and MAPBOX_ACCESS_TOKEN
  (gitignored .env, not needed by build/tests/release).
- Docs/Architecture 07 + architecture.md: build-output path corrected, marked
  outside the repository.
- 5 new tests for ue_common (54 total).

Tools/ue_import_north_buildings.py

@@ -15,6 +15,9 @@ import os
 
 import unreal
 
+sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
+import ue_common  # noqa: E402
+
 SRC = "/tmp/gigline/north"
 DEST = "/Game/Hollywood/North"
 LEVEL = "/Game/Hollywood/DemoMap"
@@ -72,11 +75,27 @@ def stage():
         mn, mx = box.min, box.max
         log(f"{label}: mesh world-space X {mn.x:.0f}..{mx.x:.0f}  Y {mn.y:.0f}..{mx.y:.0f}  size "
             f"{mx.x-mn.x:.0f}x{mx.y-mn.y:.0f} cm")
+    _make_map_writable(LEVEL)
     saved = unreal.EditorLoadingAndSavingUtils.save_map(
         unreal.EditorLevelLibrary.get_editor_world(), LEVEL)
     log(f"save_map {LEVEL}: {saved}")
+    if not ue_common.require_saved(saved, LEVEL):
+        log(f"FAILED to save {LEVEL} — staged content will not persist")
 
 
+def _make_map_writable(level: str) -> None:
+    """Clear the read-only bit an LFS .umap checks out with.
+
+    Without this `save_map()` returns False and the staged actor is lost on the
+    next load — no exception, no non-zero exit (see MEMORY: 0.1.x trap).
+    """
+    content = unreal.Paths.project_content_dir()
+    rel = level[len("/Game/"):] if level.startswith("/Game/") else level
+    umap = os.path.join(content, rel + ".umap")
+    note = ue_common.make_writable(umap)
+    if note:
+        log(f"map writable: {note}")
+
 def main():
     for fn, asset_name in IMPORTS:
         task = unreal.AssetImportTask()