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/locomotion_sandbox.gd

@@ -270,7 +270,14 @@ func _install_facing_provider() -> void:
 func _per_frame_update(delta: float) -> void:
 	if not _first_snapshot_seen:
 		return
-	greybox.set_cutaway_char_pos(player_rig.global_position)
+	# Corridor reach: how far camera-side a WALL_H wall can still occlude the
+	# character in ortho at the current (smoothed) pitch. Clamped: the -5 deg
+	# frontal preset would otherwise ask for a ~29 m corridor.
+	var pitch_abs: float = maxf(absf(camera_rig.pitch_deg()), 5.0)
+	var reach: float = clampf(
+		SandboxConstants.WALL_H / tan(deg_to_rad(pitch_abs)), 0.5, 16.0
+	)
+	greybox.set_cutaway_char_pos(player_rig.global_position, reach)
 	_anim.update(delta)