Smooth build-ghost range circle and nuke trajectory to cursor

The build-ghost icon already tracked the cursor at sub-tile precision, but
the range circle (defense post / SAM / nuke radius) and the nuke trajectory
arc still snapped to the hover tile, making them look jagged as the cursor
moved.

Range circle: cursorLoop now smooths radiusTileX/Y the same way as the icon,
except when upgrading an existing structure (the circle stays anchored to
that structure's real tile).

Nuke trajectory: split the work by cadence. The throttled renderGhost path
caches the static inputs (nearest silo + threatening SAMs) in
nukeTrajectoryStatic; cursorLoop rebuilds the Bezier each frame with the
live cursor as the destination. Source stays on the silo's tile; only the
endpoint follows the cursor.

All three previews now use the same tile-center (+0.5) convention.

src/client/controllers/BuildPreviewController.ts

@@ -52,6 +52,16 @@ export class BuildPreviewController implements Controller {
   // frame with the current cursor world position.
   private lastGhostData: GhostPreviewData | null = null;
 
+  // Static inputs for the nuke trajectory preview (source silo + threatening
+  // SAMs). Recomputed in the throttled renderGhost path; cursorLoop rebuilds
+  // the Bezier each frame with the live cursor position as the destination so
+  // the arc tracks the cursor smoothly instead of snapping tile-to-tile.
+  private nukeTrajectoryStatic: {
+    srcX: number;
+    srcY: number;
+    sams: SAMInfo[];
+  } | null = null;
+
   constructor(
     private game: GameView,
     private eventBus: EventBus,
@@ -78,16 +88,45 @@ export class BuildPreviewController implements Controller {
     // integer tile coord centers on that tile), so we subtract 0.5 here to
     // place the icon exactly under the cursor.
     const cursorLoop = () => {
-      if (this.lastGhostData !== null) {
+      const ghost = this.lastGhostData;
+      const traj = this.nukeTrajectoryStatic;
+      if (ghost !== null || traj !== null) {
         const w = this.transformHandler.screenToWorldCoordinatesFloat(
           this.mousePos.x,
           this.mousePos.y,
         );
-        this.view.updateGhostPreview({
-          ...this.lastGhostData,
-          tileX: w.x - 0.5,
-          tileY: w.y - 0.5,
-        });
+        if (ghost !== null) {
+          // The range circle (defense post / SAM / nuke radius) normally
+          // follows the cursor, so smooth it the same way as the icon. When
+          // upgrading, the circle is anchored to the existing structure's tile
+          // (stationary, correctly snapped) — leave it alone in that case.
+          const radiusFollowsCursor = !(
+            ghost.canUpgrade && ghost.upgradeTargetTile !== null
+          );
+          this.view.updateGhostPreview({
+            ...ghost,
+            tileX: w.x - 0.5,
+            tileY: w.y - 0.5,
+            ...(radiusFollowsCursor
+              ? { radiusTileX: w.x - 0.5, radiusTileY: w.y - 0.5 }
+              : {}),
+          });
+        }
+        if (traj !== null) {
+          // Rebuild the arc with the live cursor as the destination (same
+          // tile-center convention as the icon: shader adds +0.5).
+          this.view.updateNukeTrajectory(
+            buildNukeTrajectory(
+              traj.srcX,
+              traj.srcY,
+              w.x - 0.5,
+              w.y - 0.5,
+              this.game.height(),
+              this.uiState.rocketDirectionUp,
+              traj.sams,
+            ),
+          );
+        }
       }
       requestAnimationFrame(cursorLoop);
     };
@@ -228,17 +267,17 @@ export class BuildPreviewController implements Controller {
    */
   private updateNukeTrajectoryPreview(tileRef: TileRef | undefined): void {
     if (!this.ghostUnit || tileRef === undefined) {
-      this.view.updateNukeTrajectory(null);
+      this.clearNukeTrajectory();
       return;
     }
     const type = this.ghostUnit.buildableUnit.type;
     if (type !== UnitType.AtomBomb && type !== UnitType.HydrogenBomb) {
-      this.view.updateNukeTrajectory(null);
+      this.clearNukeTrajectory();
       return;
     }
     const myPlayer = this.game.myPlayer();
     if (!myPlayer) {
-      this.view.updateNukeTrajectory(null);
+      this.clearNukeTrajectory();
       return;
     }
 
@@ -246,7 +285,7 @@ export class BuildPreviewController implements Controller {
       .units(UnitType.MissileSilo)
       .filter((u) => u.isActive());
     if (silos.length === 0) {
-      this.view.updateNukeTrajectory(null);
+      this.clearNukeTrajectory();
       return;
     }
 
@@ -285,17 +324,14 @@ export class BuildPreviewController implements Controller {
       });
     }
 
-    this.view.updateNukeTrajectory(
-      buildNukeTrajectory(
-        srcX,
-        srcY,
-        dstX,
-        dstY,
-        this.game.height(),
-        this.uiState.rocketDirectionUp,
-        sams,
-      ),
-    );
+    // Stash the static inputs; cursorLoop rebuilds the Bezier each frame with
+    // the live cursor as the destination so the arc tracks smoothly.
+    this.nukeTrajectoryStatic = { srcX, srcY, sams };
+  }
+
+  private clearNukeTrajectory(): void {
+    this.nukeTrajectoryStatic = null;
+    this.view.updateNukeTrajectory(null);
   }
 
   private buildGhostPreviewData(
@@ -446,7 +482,7 @@ export class BuildPreviewController implements Controller {
     this.ghostUnit = null;
     this.lastGhostData = null;
     this.view.updateGhostPreview(null);
-    this.view.updateNukeTrajectory(null);
+    this.clearNukeTrajectory();
   }
 
   private removeGhostStructure() {