Add more test assemblies (#103)

This PR helps probe the capabilities of the engine.

Also adjusts the realization triggering system to reduce redundant realizations as we set an assembly's regulators during loading. Specificially, consolidates all calls to `realize()` into a single effect, which is triggered by the `needs_realization` signal.
Also introduces a `keep_realized` signal and use it to pause realization while loading assemblies, but this signal is planned for removal as ultimately we do not want a separate "mode" of interpreting commands during loading, for maximal reproducibility of results (and simplicity of system).

Co-authored-by: Aaron Fenyes <aaron.fenyes@fareycircles.ooo>
Reviewed-on: #103
Co-authored-by: Vectornaut <vectornaut@nobody@nowhere.net>
Co-committed-by: Vectornaut <vectornaut@nobody@nowhere.net>

app-proto/src/components/add_remove.rs

@@ -0,0 +1,54 @@
+use std::rc::Rc;
+use sycamore::prelude::*;
+
+use super::test_assembly_chooser::TestAssemblyChooser;
+use crate::{
+    AppState,
+    assembly::{InversiveDistanceRegulator, Point, Sphere}
+};
+
+#[component]
+pub fn AddRemove() -> View {
+    view! {
+        div(id="add-remove") {
+            button(
+                on:click=|_| {
+                    let state = use_context::<AppState>();
+                    state.assembly.insert_element_default::<Sphere>();
+                }
+            ) { "Add sphere" }
+            button(
+                on:click=|_| {
+                    let state = use_context::<AppState>();
+                    state.assembly.insert_element_default::<Point>();
+                }
+            ) { "Add point" }
+            button(
+                class="emoji", /* KLUDGE */ // for convenience, we're using an emoji as a temporary icon for this button
+                disabled={
+                    let state = use_context::<AppState>();
+                    state.selection.with(|sel| sel.len() != 2)
+                },
+                on:click=|_| {
+                    let state = use_context::<AppState>();
+                    let subjects: [_; 2] = state.selection.with(
+                        // the button is only enabled when two elements are
+                        // selected, so we know the cast to a two-element array
+                        // will succeed
+                        |sel| sel
+                            .clone()
+                            .into_iter()
+                            .collect::<Vec<_>>()
+                            .try_into()
+                            .unwrap()
+                    );
+                    state.assembly.insert_regulator(
+                        Rc::new(InversiveDistanceRegulator::new(subjects))
+                    );
+                    state.selection.update(|sel| sel.clear());
+                }
+            ) { "🔗" }
+            TestAssemblyChooser {}
+        }
+    }
+}