feat(director): add authored camera directions and anchors

src/scenes/cameraMotion.js

@@ -0,0 +1,70 @@
+import { sampleCameraMove } from '../director/camera.js';
+
+/** Own one authored camera animation and settle it immediately on cancellation or replacement. */
+export function createCameraMotion({
+  applyPose,
+  now = () => performance.now(),
+  requestFrame = (callback) => requestAnimationFrame(callback),
+  cancelFrame = (handle) => cancelAnimationFrame(handle),
+} = {}) {
+  let active = null;
+  let disposed = false;
+  const cancel = () => active?.finish(false);
+  return {
+    get active() {
+      return active !== null;
+    },
+    cancel,
+    destroy() {
+      disposed = true;
+      cancel();
+    },
+    play(move, token) {
+      cancel();
+      if (disposed || token?.cancelled || token?.signal?.aborted)
+        return Promise.resolve(false);
+      if (!Number.isFinite(move.durationSec) || move.durationSec <= 0)
+        throw new RangeError('Camera move duration must be positive');
+      return new Promise((resolve, reject) => {
+        const run = { handle: null, finish: null };
+        const startedAt = now();
+        const onAbort = () => run.finish(false);
+        run.finish = (completed, error) => {
+          if (active !== run) return;
+          active = null;
+          if (run.handle !== null) cancelFrame(run.handle);
+          token?.signal?.removeEventListener('abort', onAbort);
+          if (error) reject(error);
+          else resolve(completed);
+        };
+        active = run;
+        token?.signal?.addEventListener('abort', onAbort, { once: true });
+        const tick = () => {
+          if (active !== run) return;
+          run.handle = null;
+          if (disposed || token?.cancelled || token?.signal?.aborted) {
+            run.finish(false);
+            return;
+          }
+          try {
+            const progress = Math.min(
+              1,
+              Math.max(0, (now() - startedAt) / (move.durationSec * 1000)),
+            );
+            if (applyPose(sampleCameraMove(move, progress)) === false) {
+              run.finish(false);
+              return;
+            }
+            // Applying a pose can synchronously transfer camera ownership.
+            if (active !== run) return;
+            if (progress >= 1) run.finish(true);
+            else run.handle = requestFrame(tick);
+          } catch (error) {
+            run.finish(false, error);
+          }
+        };
+        tick();
+      });
+    },
+  };
+}