feat(client): 3D locomotion sandbox — character walks the live Gauntlet (T-1088)

New SR_LIVE sandbox scene: CharacterVisual composited in a 3D greybox world
derived from server snapshots. Per-leg constant-velocity interpolation keyed
to the stance throttle, 'server feet / client eyes' facing (wire octant while
moving, client aim octant idle), cadence-synced gait state machine on
AnimationPlayer custom blends, D-148 orthographic follow camera (-30deg
default, T-cycle presets), sim-space grid shader, camera-side wall cutaway,
accumulating never-evict tile store with four-state visibility tint.

Additive seams only: InputMapper.facing_angle_provider (2D path unchanged),
CharacterVisual.play_animation blend_time param + get_animation_player().
Visual harness gains per-scenario scene field + SR_AUTOPILOT input scripting.
210 new gdUnit assertions across five suites; verified live (230/230 total,
clean smoke, screenshot at .cache/screenshots/locomotion_idle_live.png).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

client/tests/visual_capture.gd

@@ -1,5 +1,6 @@
-## Visual test capture engine — boots main.tscn, runs a named scenario or flow,
-## captures viewport PNGs for golden comparison or ad-hoc inspection.
+## Visual test capture engine — boots the entry's "scene" (default main.tscn),
+## runs a named scenario or flow, captures viewport PNGs for golden comparison
+## or ad-hoc inspection.
 ##
 ## Usage:
 ##   godot --rendering-driver opengl3 --fixed-fps 60 --resolution 960x540 \
@@ -66,10 +67,22 @@ func _run():  # gdlint:disable=max-returns
 	# Ensure output directory exists
 	DirAccess.make_dir_recursive_absolute(_output_dir)
 
-	# Load main scene (autoloads already initialized from project.godot)
-	var main_scene = load("res://scenes/main.tscn")
+	# Per-entry overrides (T-1088, design §10.2 — both additive; existing entries
+	# without these keys behave byte-identically):
+	#   "scene": alternate root scene (default res://scenes/main.tscn)
+	#   "env":   env vars set before the scene boots (e.g. SR_AUTOPILOT) — OS-level,
+	#            so the scene's _ready() reads them exactly like shell-exported vars;
+	#            run-visual has no per-scenario env mechanism, this is it.
+	var entry_cfg := _entry_config()
+	var env_vars: Dictionary = entry_cfg.get("env", {})
+	for env_key: String in env_vars:
+		OS.set_environment(env_key, str(env_vars[env_key]))
+
+	# Load root scene (autoloads already initialized from project.godot)
+	var scene_path: String = entry_cfg.get("scene", "res://scenes/main.tscn")
+	var main_scene = load(scene_path)
 	if main_scene == null:
-		push_error("visual_capture: failed to load res://scenes/main.tscn")
+		push_error("visual_capture: failed to load %s" % scene_path)
 		quit(1)
 		return
 
@@ -298,6 +311,19 @@ func _capture(path: String) -> void:
 	print("visual_capture: captured -> %s (%dx%d)" % [path, image.get_width(), image.get_height()])
 
 
+## Config entry for the active scenario or flow ({} when the name is unknown —
+## the mode runners report that error themselves).
+func _entry_config() -> Dictionary:
+	var section: Dictionary = {}
+	if not _scenario.is_empty():
+		section = _config.get("scenarios", {})
+		return section.get(_scenario, {})
+	if not _flow.is_empty():
+		section = _config.get("flows", {})
+		return section.get(_flow, {})
+	return {}
+
+
 func _load_config() -> Dictionary:
 	# Config is at tests/visual.json relative to repo root.
 	# Repo root = parent of Godot project root (client/).