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>

tests/visual.json

@@ -76,6 +76,19 @@
     "atlas_gen_open": {
       "ticks": 240,
       "description": "Live: real atlas opener (HudGroups) + GJ71c Layer-1 overlays (#960)"
+    },
+    "locomotion_idle_live": {
+      "ticks": 90,
+      "live": true,
+      "scene": "res://scenes/locomotion_sandbox.tscn",
+      "description": "Live 3D sandbox: character idle at Hub spawn — greybox floor, -30 deg frame (T-1088)"
+    },
+    "locomotion_cutaway": {
+      "ticks": 540,
+      "live": true,
+      "scene": "res://scenes/locomotion_sandbox.tscn",
+      "env": { "SR_AUTOPILOT": "stance_up,south:6.0" },
+      "description": "Live 3D sandbox: autopilot walks south into a wall — cutaway stub golden (T-1088)"
     }
   },
   "flows": {
@@ -98,6 +111,23 @@
         { "action": "MoveNorth", "label": "near wall" },
         { "action": "MoveSouth", "label": "fog hiding" }
       ]
+    },
+    "locomotion_gaits": {
+      "interval": 2,
+      "live": true,
+      "scene": "res://scenes/locomotion_sandbox.tscn",
+      "env": {
+        "SR_AUTOPILOT": "east:2.0,stance_up,east:2.0,stance_up,east:2.0,stance_down,stance_down,stance_down,south:2.0,wait:1.5"
+      },
+      "description": "T-1088 stance ladder movie — NOT golden-compared. Movie mode starts no server: run `cd server && cargo run --bin settled-reach-server -- --test-mode` first, then `SR_LIVE=1 SR_PORT=9876 tests/run-visual --movie locomotion_gaits`",
+      "steps": [
+        { "action": "wait", "label": "careful gait east" },
+        { "action": "wait", "label": "walk gait east" },
+        { "action": "wait", "label": "sprint gait east" },
+        { "action": "wait", "label": "crouch gait south" },
+        { "action": "wait", "label": "settling to idle" },
+        { "action": "wait", "label": "idle rest" }
+      ]
     }
   },
   "crops": {