feat(client): Sprint 6 Touch — z-layer pipeline, cursor, fog, interactions, inventory, stance, radial

Three-scope z-layer rendering pipeline (D-049): world z:0-900 inside
CanvasGroup, insert overlay CanvasLayer 10, UI CanvasLayer 20, modal
CanvasLayer 30. Y-sort contract enforced (entities z_index=0). Reserved
ranges for VFX, airborne, lower floors documented in constants.gd.

Sprint 6 client tickets:
- #429: Cursor state machine — 4 states, 150ms transitions (D-056)
- #430: Fog shader rebuild — 5-layer fragment shader, animated noise (D-059)
- #432: Entity interaction list — vertical multi-verb, insert-styled (D-057)
- #433: World radial menu — 2 spokes, drag-release + click-click (D-058)
- #438: Inventory UI — 3x3 grid, 40x40px, 1-9 hotkeys (D-065)
- #439: Stance indicator — color-coded HUD, C/X keybinds (D-053)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/scripts/constants.gd

@@ -5,6 +5,48 @@ class_name Constants
 ## Tile size in pixels — all renderers and coordinate conversions use this.
 const TILE_SIZE: int = 32
 
+# D-049 Z-level rendering pipeline — three-scope architecture.
+# Full spec: docs/architecture/z-layer-gap-analysis.md
+#
+# WORLD SCOPE (z:-200 to z:900, inside FogGroup CanvasGroup)
+# All world content composited into one texture, then fog drawn over it.
+# Y-sort contract: all children of YSortGroup MUST use z_index = 0.
+# z_index is PRIMARY sort, y-position is SECONDARY (Godot #62715).
+const Z_FLOOR: int = 0              # Floor tiles — ground plane
+const Z_FLOOR_OBJECTS: int = 10     # Floor objects — cosmetic, ground shadows
+# z:20-99 reserved: liquid surface (z:110), surface effects
+const Z_YSORT: int = 100            # YSortGroup — furniture + entities + walls, all z:0 relative
+# z:110 reserved: liquid surface occlusion (alpha by depth)
+# z:150-199 reserved: ground VFX (smoke origins, gas pools, sparks)
+const Z_AIRBORNE: int = 200         # Projectiles, low-flying objects (scale ~1.0)
+# z:250-299 reserved: mid-air VFX (rising smoke, floating particles)
+const Z_OVERHEAD: int = 300         # Ceiling edges, upper structure, semi-transparent
+# z:325-349 reserved: ceiling VFX (smoke through ceiling)
+const Z_HIGH_AIRBORNE: int = 350    # Above-ceiling flying (scale 1.03-1.30, alpha fades)
+const Z_UPPER_CONTENT: int = 400    # Upper-floor entities (rare with fixed camera)
+# z:500-899 reserved: edge cases
+const Z_FOG: int = 900              # FogOverlay — OUTSIDE FogGroup, fog shader
+# Lower floors: z:-100 per floor (floor-1: z:-100 to z:-1, floor-2: z:-200 to z:-101)
+# z:-75 to z:-51 reserved: lower floor VFX
+#
+# INSERT SCOPE (CanvasLayer 10)
+# Bloom-rendered, not affected by fog or camera transform.
+const CANVAS_INSERT: int = 10       # CanvasLayer number for InsertOverlay
+#
+# UI SCOPE (CanvasLayer 20)
+# HUD, monologue, cursor — always visible.
+const CANVAS_UI: int = 20           # CanvasLayer number for UILayer
+#
+# MODAL SCOPE (CanvasLayer 30)
+# Full-screen overlays: pause, inventory modal, death screen.
+const CANVAS_MODAL: int = 30        # CanvasLayer number for ModalLayer
+#
+# Rendering ceiling: 10 floors (25m) above current floor.
+# Above this: no sprites, ground shadows + environmental effects only.
+const RENDER_CEILING_FLOORS: int = 10
+# Visible floor window (looking down): current - 2 floors (detailed + parallax).
+const VISIBLE_FLOOR_DEPTH: int = 2
+
 # D-033: Entity relationship color palette
 # Color represents the player's RELATIONSHIP to the entity, not an objective property.
 # Phase 1: default colors mapped by entity kind (Player/Npc/Object/Terrain).