Add unit tests for spiral shape and arrow shape tools

[tarone-saloni]

Summary

This issue tracks adding `#[cfg(test)]` test coverage to two currently untested shape tool files. These follow the same patterns already established in `ellipse_shape.rs`, `line_shape.rs`, and `shape_utility.rs`.

---

Files to Test

1. `editor/src/messages/tool/common_functionality/shapes/spiral_shape.rs` (~204 LOC)

What to test:

• `draw_spiral_simple` — drag with `ToolType::Shape` (spiral mode), assert a layer is created and `outer_radius` matches the drag distance
• `draw_spiral_logarithmic` — verify `inner_radius` defaults to `0.1` for `SpiralType::Logarithmic` (set in `Spiral::create_node`, line 102)
• `draw_spiral_archimedean` — verify `inner_radius` defaults to `0.0` for `SpiralType::Archimedean`
• `draw_spiral_cancel_rmb` — use `drag_tool_cancel_rmb`, assert no layer created
• `spiral_decrease_turns` — call `Spiral::update_turns(true, ...))` and assert turns clamps to minimum of `1.0`
• `spiral_increase_turns` — call `Spiral::update_turns(false, ...)` and assert turns increments by `1.0`

Key parameters to assert (via `extract_spiral_parameters`):

• `spiral_type: SpiralType`
• `inner_radius: f64`
• `outer_radius: f64` — should equal drag distance from origin
• `turns: f64`

Reference pattern: `ellipse_shape.rs`; use `NodeGraphLayer::find_node_inputs` with `graphene_std::vector::generator_nodes::spiral::IDENTIFIER`

---

2. `editor/src/messages/tool/common_functionality/shapes/arrow_shape.rs` (~116 LOC)

What to test:

• `draw_arrow_simple` — drag with `ToolType::Line` (arrow mode), assert a layer is created and `arrow_to` vector matches the drag delta
• `draw_arrow_cancel_rmb` — use `drag_tool_cancel_rmb`, assert no layer created
• `draw_arrow_snap_angle` — drag with `ModifierKeys::SHIFT` (snap angle), assert the `arrow_to` direction is constrained to a 15° multiple
• `draw_arrow_zero_length` — drag start == end, assert no crash and no layer with non-zero `arrow_to` (guarded by the `< 1e-6` check at line 59)

Key parameters to assert:

• `arrow_to: DVec2` — retrieved via `NodeGraphLayer::find_node_inputs` with `graphene_std::vector_nodes::arrow::IDENTIFIER`, input index `1`

Reference pattern: `line_shape.rs` (shares `generate_line` logic); use same `NodeGraphLayer` approach

---

Test Infrastructure

All tests should use the existing `EditorTestUtils` helper:

```rust
#[cfg(test)]
mod test_spiral {
use crate::test_utils::test_prelude::*;

#[tokio::test]
async fn draw_spiral_simple() {
    let mut editor = EditorTestUtils::create();
    editor.new_document().await;
    editor.drag_tool(ToolType::Shape, 0., 0., 40., 0., ModifierKeys::empty()).await;

    // Assert one layer created
    assert_eq!(editor.active_document().metadata().all_layers().count(), 1);

    // Extract outer_radius and assert it equals ~40.0
    // use extract_spiral_parameters or NodeGraphLayer::find_node_inputs with spiral::IDENTIFIER
}

}
```

Related files for reference:

• `ellipse_shape.rs` — drag + assert pattern
• `line_shape.rs` — angle-snapping test pattern
• `shape_utility.rs` — `extract_spiral_parameters` helper
• `graph_modification_utils.rs` — `get_spiral_id`, `get_arrow_id`