Implement Phase 17a Prop Roles UI/UX Specification and associated functionality

- Added UI/UX specification for prop roles and set down mechanics in phase 17a.
- Created `PropStageRoles` script to manage normalized behavior for props, including depth, motion, and collision settings.
- Introduced `StagePhysicsPolicy` for shared physics configurations related to props and actors.
- Developed comprehensive tests for prop roles, including default settings, legacy migration, serialization, and runtime behavior.
- Ensured proper handling of set down actions, including clearance and orientation preservation.

sandbox_theme.json

@@ -120,6 +120,64 @@
     "drag_threshold": 8,
     "selected_border_width": 3
   },
+  "stage_panels": {
+    "shell": {
+      "preferred_width": 380,
+      "edge_inset": 12,
+      "padding": 12,
+      "header_gap": 8,
+      "section_gap": 12,
+      "corner_radius": 10,
+      "surface_color": "#252B38FA",
+      "border_color": "#FFFFFF33"
+    },
+    "type": {
+      "title_size": 24,
+      "section_size": 20,
+      "body_size": 20,
+      "helper_size": 16,
+      "text_color": "#F5F7FA",
+      "muted_text_color": "#AAB2C0"
+    },
+    "choice": {
+      "minimum_height": 56,
+      "row_gap": 6,
+      "neutral_surface": "#171B24",
+      "hover_surface": "#303A4B",
+      "selected_surface": "#12394A",
+      "disabled_surface": "#252B38",
+      "border_color": "#FFFFFF33"
+    },
+    "toggle": {
+      "minimum_height": 64,
+      "help_minimum_height": 72,
+      "row_padding": 12,
+      "switch_width": 64,
+      "switch_height": 34,
+      "knob_size": 26,
+      "neutral_surface": "#202633",
+      "hover_surface": "#2B3442",
+      "selected_surface": "#173443",
+      "disabled_surface": "#252B38",
+      "border_color": "#FFFFFF33",
+      "switch_on_color": "#22C6FF",
+      "switch_off_color": "#4A5362",
+      "switch_disabled_color": "#3B414D",
+      "knob_color": "#F5F7FA"
+    },
+    "action": {
+      "min_target_size": 48,
+      "close_width": 88,
+      "surface_color": "#171B24",
+      "hover_surface": "#303A4B",
+      "selected_surface": "#12394A"
+    },
+    "prop": {
+      "stickman_cue_color": "#F5F7FA",
+      "prop_cue_color": "#B7824D",
+      "cue_outline_color": "#171B24"
+    }
+  },
   "overlays": {
     "style_defaults": { "line_width": 2, "dash_length": 8, "dash_gap": 5 },
     "terrain_guide": { "color": "#22C6FF" },
@@ -127,7 +185,12 @@
     "completed_path_color": "#959EAD",
     "valid": { "color": "#4DE66680" },
     "same_type": { "color": "#FFFFFF26" },
-    "invalid": { "color": "#FF4D4D80" }
+    "invalid": { "color": "#FF4D4D80" },
+    "set_down": {
+      "unknown_color": "#C5CAD4",
+      "reach_color": "#FFB300",
+      "ghost_alpha": 0.36
+    }
   },
   "widgets": {
     "swatches": [