Integrate the Blender character, gear and prop library

NpcAvatar
- select one of 8 library character variants by role/seed (with npc_base and the
  procedural rig as fallbacks), keep Rig/Head and the existing bone animation
- tint only skin/cloth/pants/accessory/hair; fixed detail materials keep their
  colour, npc_vest keeps its hi-vis vest, firefighter/guard use dark uniform tints
- attach caps, hoods, helmets, backpacks, SCBA tanks, lanyards and hand items at
  the documented head/back/chest/hand points (primitive fallbacks retained)

WorldBuilder
- prefer assets/models/blender/props/ for the six replacements and the new
  street dressing (street lamp/tree, palms, planters, A-frame, cone, barrier,
  stanchion, podium, handrail, wall lantern, double door, marquee, sedan)
- fit the same colliders/navigation obstacles and keep the placement validator;
  collect library emissives (bulbs, marquee face, wall lanterns, door LED, sedan
  lights) for set_night_mode()
- add a library triangle counter + 60k budget check (current ~27,986); toggle
  prop gallery to the library models; fix the marquee label placement

Tests/smoke
- character selection/gear attach/fixed-material tint tests; library prop load,
  emissive and triangle-budget tests; smoke_props routes a guest around a
  stanchion in open sidewalk (the old bench route is now crowded by trees)
- headless 222/222, Python 68/68, all smokes pass, compare 11 good / 0 bad

Docs: TODO Phase 3.5 moved to DONE, README/architecture/blender-assets/plan
describe the integrated runtime, MEMORY records the integration rules.

src/test/capture_prop_gallery.gd

@@ -7,6 +7,7 @@ extends SceneTree
 
 const OUT_DIR := "user://screenshots"
 const MODEL_DIR := "res://assets/models/generated/"
+const LIBRARY_DIR := "res://assets/models/blender/props/"
 const PROPS := ["bollard", "bench", "trash_bin", "planter_box",
 	"newspaper_box", "fire_hydrant"]
 const SPACING := 1.9
@@ -74,7 +75,10 @@ func _build_stage() -> void:
 	var columns := [-1.95, 0.0, 1.95]
 	for index in PROPS.size():
 		var prop_name: String = PROPS[index]
-		var packed := ResourceLoader.load(MODEL_DIR + prop_name + ".glb") as PackedScene
+		var path := LIBRARY_DIR + prop_name + ".glb"
+		if not ResourceLoader.exists(path):
+			path = MODEL_DIR + prop_name + ".glb"
+		var packed := ResourceLoader.load(path) as PackedScene
 		if packed == null:
 			continue
 		var model := packed.instantiate() as Node3D