fix(client): cutaway cuts the camera-character sightline corridor, not a proximity dome (T-1088)

Live-session finding: the half-disc cut bit nearby side walls while leaving
the wall actually occluding the character at full height, with dome-contour
scallops across the two-row walls. Rewritten as a sightline corridor: walls
within CUT_CORRIDOR_HALF_W of the character's x, camera-side, out to a
pitch-derived reach (WALL_H / tan(pitch) — how far a wall can still occlude
in ortho; ~4.3 m at -30, breathes with the T-cycle tilt) drop to the stub.
FollowCamera3D exposes pitch_deg(); reach clamped 0.5-16 m.

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

client/scripts/sandbox/follow_camera_3d.gd

@@ -96,6 +96,12 @@ func _process(delta: float) -> void:
 	_apply_orbit()
 
 
+## Current (smoothed) pitch in degrees from horizontal — consumed by the
+## cutaway corridor reach (sandbox root pushes WALL_H / tan(pitch) per frame).
+func pitch_deg() -> float:
+	return _pitch_deg
+
+
 # Dev affordances only — there is deliberately NO yaw/rotation input path here.
 func _unhandled_input(event: InputEvent) -> void:
 	if event is InputEventKey: