feat(client): cutaway defaults to mode 2 (both) — user verdict from the live session (T-1088)

Known interiors open at a glance (bottom walls low) AND the sightline
corridor punches through unexplored exterior walls; C still cycles modes.
sandbox_constants.gd also carries the path-preview/gesture knobs for the
click-to-move feature landing in the next commit (single tuning file).

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

client/scripts/sandbox/greybox_world.gd

@@ -196,6 +196,7 @@ func _ready() -> void:
 	_wall_material.set_shader_parameter("u_corridor_half_w", SandboxConstants.CUT_CORRIDOR_HALF_W)
 	_wall_material.set_shader_parameter("u_cut_band", SandboxConstants.CUT_BAND)
 	_wall_mm = _make_layer("WallMM", wall_mesh, _wall_material, true)
+	set_cutaway_mode(SandboxConstants.CUTAWAY_MODE)
 	# Camera-far neighbor directions for the bottom-wall rule — derived from the
 	# actual WorldRoot basis at runtime (never hand-derived; the E/W-mirror
 	# lesson). A wall is a "bottom wall" when a KNOWN non-wall tile sits on its

client/scripts/sandbox/sandbox_constants.gd

@@ -24,6 +24,10 @@ const CUT_HEIGHT := 0.75
 const CUT_CORRIDOR_HALF_W := 1.4
 ## Cutaway smoothstep falloff band (m) at the corridor edges.
 const CUT_BAND := 0.75
+## Cutaway mode default: 0 = sightline corridor, 1 = bottom walls low, 2 = both.
+## User verdict (live session 2026-07-06): "both" — known interiors open at a
+## glance AND the corridor punches through unexplored exterior walls. C cycles.
+const CUTAWAY_MODE := 2
 
 # -- Locomotion interpolation (design §4) --------------------------------------
 
@@ -116,3 +120,18 @@ const TINTS := {
 
 ## Ground-plane deadzone (m) for the mouse facing provider — mirrors the 2D jitter guard.
 const MOUSE_AIM_DEADZONE_M := 0.1
+
+# -- Path preview / click-to-move (T-1088 live-session item 2) ------------------------
+
+## Move-here marker fill on the hovered destination subtile — distinct accent,
+## semi-opaque so the floor tint reads through.
+const PATH_MARKER_COLOR := Color(0.30, 0.90, 1.0, 0.55)
+## Optimal-path line colour (tile-center strip from the character to the destination).
+const PATH_LINE_COLOR := Color(0.30, 0.90, 1.0, 0.90)
+## Marker sits this far above the floor plane (m) — clears grid-line z-fighting.
+const PATH_MARKER_Y := 0.02
+## Path line sits slightly higher than the marker so it reads on top of it.
+const PATH_LINE_Y := 0.06
+## RMB hold (ms) past which a right-click is a LONG-PRESS = go-there-then-crouch
+## (committed on release), below which it is the click/double-click gesture.
+const RMB_LONG_PRESS_MS := 400