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.

scripts/terrain_utils.gd

@@ -0,0 +1,104 @@
+class_name TerrainUtils
+extends RefCounted
+## TerrainUtils - Static geometry engine for vector terrain (Phase 3).
+##
+## Two responsibilities: sanitize raw input points into a clean, grid-snapped,
+## clockwise-ordered polygon; and spawn a configured TerrainBlock into a target
+## container. Consumed by the standalone physics test harness; not referenced by
+## the editor.
+
+# ---------------------------------------------------------------------------
+# Constants
+# ---------------------------------------------------------------------------
+
+const DEFAULT_GRID_SIZE: float = 16.0
+
+const DEFAULT_FILL_COLOR := Color(0.25, 0.55, 0.25, 1.0)
+const DEFAULT_OUTLINE_COLOR := Color(0.05, 0.10, 0.05, 1.0)
+const DEFAULT_OUTLINE_WIDTH: float = 2.0
+
+# ---------------------------------------------------------------------------
+# Point sanitization
+# ---------------------------------------------------------------------------
+
+## Snap raw points to a grid, drop redundant nodes, and enforce clockwise
+## winding (required for solid collision decomposition). Returns a new array;
+## the input is not modified.
+static func sanitize_points(points: PackedVector2Array, grid_size: float = DEFAULT_GRID_SIZE) -> PackedVector2Array:
+	var cleaned := PackedVector2Array()
+	for p: Vector2 in points:
+		cleaned.append(_snap_to_grid(p, grid_size))
+	cleaned = _simplify_polyline(cleaned)
+	if cleaned.size() >= 3 and not Geometry2D.is_polygon_clockwise(cleaned):
+		cleaned.reverse()
+	return cleaned
+
+# ---------------------------------------------------------------------------
+# Factory spawner
+# ---------------------------------------------------------------------------
+
+## Sanitize `points`, create a new TerrainBlock, apply the cleaned points and
+## styling, and add it to `container`. Returns the spawned block.
+static func spawn_block(
+	container: Node,
+	points: PackedVector2Array,
+	grid_size: float = DEFAULT_GRID_SIZE,
+	fill_color: Color = DEFAULT_FILL_COLOR,
+	outline_color: Color = DEFAULT_OUTLINE_COLOR,
+	outline_width: float = DEFAULT_OUTLINE_WIDTH
+) -> TerrainBlock:
+	var block := TerrainBlock.new()
+	block.name = "TerrainBlock"
+	block.polygon_points = sanitize_points(points, grid_size)
+	block.fill_color = fill_color
+	block.outline_color = outline_color
+	block.outline_width = outline_width
+	container.add_child(block)
+	return block
+
+# ---------------------------------------------------------------------------
+# Internal helpers
+# ---------------------------------------------------------------------------
+
+static func _snap_to_grid(p: Vector2, grid_size: float) -> Vector2:
+	if grid_size <= 0.0:
+		return p
+	return Vector2(roundf(p.x / grid_size) * grid_size, roundf(p.y / grid_size) * grid_size)
+
+
+## Remove redundant nodes: consecutive duplicates (e.g. collapsed by grid
+## snapping) and collinear middle vertices (redundant for both render and
+## collision). NOTE: Geometry2D.simplify_polyline() does not exist in Godot
+## 4.7.1, so this local pass stands in for it.
+static func _simplify_polyline(points: PackedVector2Array) -> PackedVector2Array:
+	var deduped := PackedVector2Array()
+	for p: Vector2 in points:
+		if not deduped.is_empty() and deduped[deduped.size() - 1].is_equal_approx(p):
+			continue
+		deduped.append(p)
+
+	# Drop a closing duplicate (last vertex == first vertex). Polygons are
+	# treated as open here; otherwise the collinear pass below would treat the
+	# first/last vertices as "collinear" with each other and erase the first
+	# corner, producing a degenerate polygon.
+	if deduped.size() >= 2 and deduped[0].is_equal_approx(deduped[deduped.size() - 1]):
+		deduped.remove_at(deduped.size() - 1)
+
+	if deduped.size() < 3:
+		return deduped
+
+	var simplified := PackedVector2Array()
+	var n: int = deduped.size()
+	for i: int in n:
+		var prev := deduped[(i - 1 + n) % n]
+		var curr := deduped[i]
+		var next := deduped[(i + 1) % n]
+		if _is_collinear(prev, curr, next):
+			continue
+		simplified.append(curr)
+
+	return simplified if simplified.size() >= 3 else deduped
+
+
+static func _is_collinear(a: Vector2, b: Vector2, c: Vector2) -> bool:
+	return absf((b - a).cross(c - b)) < 0.001