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/tests/test_generated_props.gd

@@ -201,6 +201,38 @@ func test_fallback_props_get_the_same_collider() -> bool:
 	root.free()
 	return a and b and c and d
 
+func test_world_loads_the_blender_library_props() -> bool:
+	var root := Node3D.new()
+	var builder := WorldBuilder.new(root)
+	builder.build()
+	var a := TestUtils.check(builder.fallback_props.is_empty(),
+		"no primitive prop fallback (fallbacks: %s)" % str(builder.fallback_props))
+	var b := TestUtils.check(builder.library_props.has("street_lamp")
+		and builder.library_props.has("street_tree")
+		and builder.library_props.has("bollard")
+		and builder.library_props.has("palm_tree")
+		and builder.library_props.has("sedan"),
+		"replacement and new props come from the library (%d)" % builder.library_props.size())
+	var c := TestUtils.check(root.get_node_or_null("Building/LibraryDoubleDoor") != null
+		and root.get_node_or_null("LibraryMarquee") != null
+		and root.get_node_or_null("ParkedSedan") != null,
+		"library door, marquee and sedan are placed")
+	var d := TestUtils.check(not builder._lamp_bulb_mats.is_empty()
+		and not builder._marquee_emissive.is_empty(),
+		"library lamps/marquee expose emissive materials")
+	builder.set_night_mode(true)
+	var lit := (builder._lamp_bulb_mats[0].emission_energy_multiplier > 0.0
+		and builder._marquee_emissive[0].emission_energy_multiplier > 0.0)
+	builder.set_night_mode(false)
+	var dark := builder._lamp_bulb_mats[0].emission_energy_multiplier <= 0.0
+	var e := TestUtils.check(lit and dark, "night mode drives library emissives")
+	var triangles := builder.library_triangle_count()
+	var f := TestUtils.check(triangles > 0
+		and triangles <= WorldBuilder.LIBRARY_TRIANGLE_BUDGET,
+		"library prop triangles stay inside the budget (%d)" % triangles)
+	root.free()
+	return a and b and c and d and e and f
+
 func test_world_facade_windows_light_at_night() -> bool:
 	var root := Node3D.new()
 	var builder := WorldBuilder.new(root)