feat: Add project roadmap and initial specifications for dynamic vector props and terrain system

- Created ROADMAP.md outlining core features, implementation phases, and detailed breakdown for the Stickman Sandbox Builder project.
- Introduced plans for dynamic vector props with `PropBlock` specification, including reusable components and utility functions for prop creation.
- Developed a vector terrain system plan detailing the reusable `TerrainBlock` component and associated utility functions for geometry handling.
- Implemented `PhysicsTestHarness` scene for testing physics interactions with dynamic props and terrain.
- Added scripts for `PropBlock`, `PropUtils`, `TerrainBlock`, and `TerrainUtils` to support dynamic prop creation and terrain management.

AGENTS.md

@@ -365,6 +365,63 @@ assembled in a "Whole Stickman" preview that supports translation, rotation, and
     persistence to disk. The "Facing" menu and all animation controls are **hidden until an .stk is
     loaded** (`_set_rig_controls_visible(false)` at the end of `_build_ui()` and in
     `_free_current_rig()`; shown on successful spawn in `_load_and_spawn()`).
+- `scripts/terrain_block.gd` — `class_name TerrainBlock`, `extends StaticBody2D`; a **reusable
+  vector terrain component** (Vector Terrain System, **not used by the editor**). Builds its three
+  children in code: `Polygon2D` (interior fill, `fill_color`), `Line2D` (crisp vector border,
+  `outline_color`/`outline_width`, auto-closed loop by appending the first vertex to the end,
+  `LINE_JOINT_ROUND` + round caps), and `CollisionPolygon2D` (`BUILD_SOLIDS` solid decomposition —
+  supports concave blocks). Exported properties: `polygon_points: PackedVector2Array`,
+  `fill_color`, `outline_color`, `outline_width`; a unified setter pushes vertex changes to all
+  three children live (no manual rebuilds).
+- `scripts/terrain_utils.gd` — `class_name TerrainUtils`, `extends RefCounted`; static utility
+  (**not used by the editor**):
+  - `sanitize_points(points: PackedVector2Array, grid_size: float = 16.0) -> PackedVector2Array` —
+    sanitization pipeline in order: grid snap → redundancy removal via a **local
+    `_simplify_polyline()`** (Godot 4.7 has **no `Geometry2D.simplify_polyline()`**; the local
+    version drops consecutive duplicates, a closing duplicate when `last == first`, and collinear
+    vertices) → clockwise enforcement via `Geometry2D.is_polygon_clockwise()` (reverses if false,
+    guaranteeing clockwise output).
+  - `spawn_block(...)` — factory that sanitizes raw input vectors (`sanitize_points`), creates a
+    `TerrainBlock`, applies the cleaned points, and adds it to the target container.
+- `scripts/prop_block.gd` — `class_name PropBlock`, `extends RigidBody2D`; a **reusable
+  dynamic vector prop component** (Dynamic Vector Props, **not used by the editor**), `@tool`.
+  Builds its children in code: `Polygon2D` (interior fill, `fill_color`), `Line2D` (crisp
+  vector outline, `outline_color`/`outline_width`, auto-closed loop by appending the first
+  vertex, `LINE_JOINT_ROUND` + `LINE_CAP_ROUND`), and a collision node — `CollisionPolygon2D`
+  (`BUILD_SOLIDS`) in `POLYGON` mode, or `CollisionShape2D` + `CircleShape2D` (48-segment
+  radial loop, `CIRCLE_SEGMENTS = 48`) in `CIRCLE` mode, toggled via `shape_type`. Exported
+  properties: `shape_type` (`@export_enum("Polygon","Circle")`), `polygon_points:
+  PackedVector2Array`, `radius: float`, `fill_color`, `outline_color`, `outline_width`, and
+  `material_preset` (`@export_enum("None","Wood","Rubber","Cardboard","Metal")`). Presets set
+  `mass` + `physics_material_override` via static `mass_for()`/`physics_material()`/`tint_for()`
+  (Wood: mass 3.0, friction 0.6, bounce 0.1; Rubber: mass 0.5, friction 0.9, bounce 0.85;
+  Cardboard: mass 0.4, friction 0.3, bounce 0.05; Metal: mass 8.0, friction 0.9, bounce 0.0;
+  None: mass 1.0, friction 0.5, bounce 0.05) and recolor fill/outline for non-`NONE`. Unified
+  live-update setters push geometry/color changes to all children (null-guarded for `@tool`
+  editor safety); `_apply_shape()` enables exactly one collision node.
+- `scripts/prop_utils.gd` — `class_name PropUtils`, `extends RefCounted`; static factory
+  (**not used by the editor**):
+  - `create_box(size := Vector2(48,48))` / `create_ball(radius := 24.0)` /
+    `create_plank(length := 160.0, thickness := 16.0)` / `create_triangle(base := 56.0, height
+    := 48.0)` — primitive generators returning shape-payload dictionaries with default
+    dimensions + color themes (wood/rubber/metal/cardboard).
+  - `spawn_prop(container, position, shape_payload, material_preset := WOOD, initial_velocity
+    := Vector2.ZERO) -> PropBlock` — instantiates a `PropBlock`, applies the payload
+    (`shape_type` + geometry + colors via `_apply_shape_payload()`, sanitizing polygon points
+    through `TerrainUtils.sanitize_points()`), sets `linear_velocity` after `add_child` (only
+    when non-zero), and returns the spawned prop.
+- `scripts/physics_test_harness.gd` — `class_name PhysicsTestHarness`, `extends Node2D`; standalone
+  staging scene root (Vector Terrain System / Dynamic Vector Props, **not wired into the editor**;
+  run via **F6** on `res://scenes/physics_test_harness.tscn`). Builds flat ground, angled ramps, and stepped
+  `TerrainBlock` instances via `TerrainUtils`, instantiates `res://master_rig.tscn` standing on the
+  flat ground, and handles camera input. **Keys 1/2/3** spawn dynamic props above the angled ramp
+  via `PropUtils.spawn_prop()` (`PROP_SPAWN_POSITION = (300, -300)`): **1** Wood Crate
+  (`create_box()`, `WOOD`, velocity `(60,0)`), **2** Bouncy Ball (`create_ball()`, `RUBBER`,
+  `(-80,0)`), **3** Heavy Plank (`create_plank()`, `METAL`, `(30,-40)`). Adds a **best-effort
+  `StaticBody2D` collision proxy** (`RigCollisionProxy`, `_add_rig_collision_proxy()`) since the
+  rig has **no physics bodies of its own** — a 240×1000 px `RectangleShape2D` centered at `(0,-500)`
+  (`RIG_PROXY_SIZE`/`RIG_PROXY_CENTER`) matching the standing figure's world bounds, so props
+  bounce/rest against it; the proxy is a code-only stand-in, not part of the rig.
 - Scenes:
   - `scenes/stickman_editor.tscn` — main editor layout; unique-name nodes (`%Prefix`) used
     for typed `@onready` access: `%MenuBar`, `%StickmanNameEdit`, `%LeftColumn`,
@@ -382,6 +439,13 @@ assembled in a "Whole Stickman" preview that supports translation, rotation, and
     loaded-filename status label;
     `SubViewport` world with an enabled `Camera2D` (middle-mouse pan, wheel zoom, recenter on
     spawn); interactive limb IK via `SkeletonModificationStack2D` TwoBoneIK.
+  - `scenes/physics_test_harness.tscn` — **standalone staging scene** (Vector Terrain System /
+    Dynamic Vector Props, not wired into the editor; run via **F6**). Backed by
+    `scripts/physics_test_harness.gd`. Root `Node2D` + script, `Camera2D` at position `(0, -400)`
+    zoom `0.5`, empty Environment container. Wheel-zoom scales between `0.25x` and `3.0x`
+    (resolution independence / vector outline thickness); middle-drag pans. Keys **1/2/3** spawn
+    dynamic props (`PropUtils`) above the angled ramp; a best-effort `StaticBody2D` rig collision
+    proxy provides a surface for props to bounce/rest against.
 
 ### Body-part data model
 - 10 internal part keys (ordered): `head`, `torso`, `left_upper_arm`, `left_lower_arm`,