Add scripted lightning weapon mods with interpolated anchored effects

mods/lightning_shield/scripts/equipment.rhai

@@ -0,0 +1,6 @@
+// Pickups use the same reusable equipment as the shop.
+fn on_pickup(event, state, random) {
+    let recovered = if state.contains("recovered") { state.recovered % 100000 + 1 } else { 1 };
+    #{ commands: [#{ op: "objective", key: "lightning_shield:recovered", value: recovered }],
+       state: #{ recovered: recovered } }
+}

mods/lightning_shield/scripts/weapon.rhai

@@ -0,0 +1,70 @@
+// This is an ordinary non-special weapon: it runs only while Fire is held.
+// Measure range to the nearest point of an enemy hull, not its center, so large
+// ships can be zapped at the edge too. The reach is only 12px past our 32px hull.
+fn on_fire(event, state, random) {
+    if event.hostile {
+        return #{ commands: [], state: state };
+    }
+    let bursts = if state.contains("bursts") { state.bursts % 100000 + 1 } else { 1 };
+    let hits = if state.contains("hits") { state.hits } else { 0 };
+    let commands = [];
+    let struck = [];
+    let from_x = event.player.x + 16;
+    let from_y = event.player.y + 16;
+
+    // An electric halo just outside the hull follows the ship throughout the
+    // recharge. Both endpoints use the public arc anchoring API; it is visual
+    // only, so even an empty pulse still lights the ring without playing audio.
+    let ring = [[19, 0], [18, 7], [13, 13], [7, 18], [0, 19], [-7, 18],
+                [-13, 13], [-18, 7], [-19, 0], [-18, -7], [-13, -13],
+                [-7, -18], [0, -19], [7, -18], [13, -13], [18, -7]];
+    for i in 0..ring.len() {
+        let a = ring[i];
+        let b = ring[(i + 1) % ring.len()];
+        commands.push(#{ op: "arc", x: from_x + a[0], y: from_y + a[1],
+                         to_x: from_x + b[0], to_y: from_y + b[1],
+                         anchor: 0, to_anchor: 0, ticks: 8 });
+    }
+
+    // Bound each pulse's work and sound: at most four close airborne ships.
+    for target in 0..4 {
+        let selected = #{};
+        let nearest = 1000000;
+        for enemy in event.entities {
+            if enemy.health <= 0 || enemy.ground || struck.contains(enemy.entity) {
+                continue;
+            }
+            let x = max(enemy.x, min(from_x, enemy.x + enemy.width - 1));
+            let y = max(enemy.y, min(from_y, enemy.y + enemy.height - 1));
+            if x < 0 || x >= 320 || y < 0 || y >= 200 { continue; }
+            let dx = x - from_x;
+            let dy = y - from_y;
+            let distance = dx * dx + dy * dy;
+            if distance <= 28 * 28 && distance < nearest {
+                selected = #{ entity: enemy.entity, x: x, y: y };
+                nearest = distance;
+            }
+        }
+        if !selected.contains("entity") { break; }
+        // Start at the halo point facing this hull, not at the player's center.
+        let dx = selected.x - from_x;
+        let dy = selected.y - from_y;
+        let origin = ring[0];
+        let best = -1000000;
+        for point in ring {
+            let facing = point[0] * dx + point[1] * dy;
+            if facing > best { best = facing; origin = point; }
+        }
+        commands.push(#{ op: "arc", x: from_x + origin[0], y: from_y + origin[1],
+                         to_x: selected.x, to_y: selected.y,
+                         anchor: 0, to_anchor: selected.entity, ticks: 3 });
+        commands.push(#{ op: "damage", entity: selected.entity, amount: 2 });
+        struck.push(selected.entity);
+        hits = hits % 100000 + 1;
+    }
+    if struck.len() > 0 {
+        commands.push(#{ op: "play_sound", sound: "lightning_shield:fire" });
+        commands.push(#{ op: "play_sound", sound: "lightning_shield:impact" });
+    }
+    #{ commands: commands, state: #{ bursts: bursts, hits: hits } }
+}