npc: AGigLineNPCPerson prefab + stable-named screenshots

- New C++ AGigLineNPCPerson: reusable NPC/pedestrian prefab built on the MPFB
  human. ACharacter capsule half-height 83 cm (total 1.66 m, head top ~1.66 m
  so a 1.60 m camera doesn't clip), the character skeletal mesh on the built-in
  mesh as the visible body (feet on Z 0), AutoPossessAI for 0.5.0 pedestrians.
  Staged at Z 0 in the review scene; captured as review-npc-prefab.png.
- AGigLineShotActor now names screenshots by stable camera name (no list-index
  prefix): inserting or reordering cameras no longer renumbers files or orphans
  the old ones. Re-rendered all review/demo shots with stable names.
- ue_build_review_map.py: chmod the target .umap before save_map (git marks maps
  444 on checkout, silently making save_map return False and dropping stages).
- Screenshots/README.md, ROADMAP, TODO updated for the prefab + stable names.
- DemoMap.umap rebuilt by the render pipeline (north buildings + demo cameras).

Tools/ue_build_review_map.py

@@ -25,6 +25,20 @@ def log(msg):
     lines.append(str(msg))
 
 
+def ensure_writable(path):
+    # A packed .umap left read-only (git sets 644->444 on checkout, or the
+    # editor re-flags it) makes save_map silently return False with no error.
+    # chmod u+w it so the staged camera list and actors actually persist.
+    p = os.path.join(unreal.Paths.project_dir(), path.replace("/Game/", "Content/"))
+    try:
+        mode = os.stat(p).st_mode & 0o777
+        if mode & 0o200 == 0:
+            os.chmod(p, 0o644)
+            log(f"map writable: chmod u+w {p} (was {oct(mode)})")
+    except FileNotFoundError:
+        pass
+
+
 def spawn(cls, loc, label=None, scale=None, rot=None):
     a = unreal.EditorLevelLibrary.spawn_actor_from_class(cls, loc)
     if a is None:
@@ -149,6 +163,18 @@ try:
     else:
         log("MISSING: /Script/GigLine.GigLineCharacter")
 
+    # --- NPC prefab -----------------------------------------------------------
+    # The reusable NPC person: a collision capsule + visible MPFB body, standing
+    # feet on Z 0, placed clear of the pawn so the prefab reads on its own.
+    npc = None
+    npc_class = unreal.load_class(None, "/Script/GigLine.GigLineNPCPerson")
+    if npc_class:
+        npc = spawn(npc_class, unreal.Vector(-4500, 300, 0), "NPC_Person")
+        if npc:
+            log("staged: NPC_Person (AGigLineNPCPerson) at Z 0")
+    else:
+        log("MISSING: /Script/GigLine.GigLineNPCPerson")
+
     # --- camera list ---------------------------------------------------------
     # Every camera is a (name, location, target) triple; the look-at rotation is
     # computed from the pair so a camera cannot drift out of aim when one number
@@ -162,6 +188,9 @@ try:
         ("scale-cubes", unreal.Vector(0, -900, 200), unreal.Vector(0, 600, 80)),
         # the pawn the game spawns, with the grounded cube beside it
         ("player-placement", unreal.Vector(320, -520, 140), unreal.Vector(0, 0, 80)),
+        # the NPC prefab: capsule + visible body, feet on Z 0, at double scale
+        # distance because it stands off to the west of the main staging
+        ("npc-prefab", unreal.Vector(-4200, 600, 160), unreal.Vector(-4500, 300, 90)),
         # low, close on feet and cube base: ground contact must be obvious
         ("ground-contact", unreal.Vector(180, -260, 35), unreal.Vector(0, 40, 20)),
         # shadow comparison, low angle so both shadows read
@@ -214,6 +243,7 @@ try:
             # quit after the demo shots so a dedicated DemoMap capture run
             # terminates cleanly (render_screenshots.sh runs both maps).
             seq.set_editor_property("quit_when_done", True)
+        ensure_writable("/Game/Hollywood/DemoMap")
         saved = unreal.EditorLoadingAndSavingUtils.save_map(
             unreal.EditorLevelLibrary.get_editor_world(), "/Game/Hollywood/DemoMap")
         log(f"demo map staged: {len(demo_shots)} cameras, save {saved}")
@@ -239,6 +269,7 @@ except Exception as exc:  # noqa
 # --- save --------------------------------------------------------------------
 
 try:
+    ensure_writable(MAP_PATH)
     world = unreal.EditorLevelLibrary.get_editor_world()
     saved = unreal.EditorLoadingAndSavingUtils.save_map(world, MAP_PATH)
     log(f"save_map {MAP_PATH}: {saved}")