feat(physics): PositionManager combiner class + 6 unit tests (L.3.2)

Pure-function ComputeOffset(dt, pos, seqVel, ori, interp, maxSpeed) →
Vector3. Combines animation root motion (seqVel × dt rotated by body
orientation) with InterpolationManager.AdjustOffset world-space
correction. Mirrors retail CPhysicsObj::UpdateObjectInternal
(acclient @ 0x00513730).

Composed into RemoteMotion in subsequent task (L.3.1+L.3.2 Task 3);
not yet consumed.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/AcDream.Core/Physics/PositionManager.cs

@@ -0,0 +1,55 @@
+using System.Numerics;
+
+namespace AcDream.Core.Physics;
+
+/// <summary>
+/// Per-frame combiner for remote-entity motion: animation root motion
+/// + InterpolationManager catch-up correction. Pure function — no
+/// side effects, no hidden state.
+///
+/// Mirrors retail CPhysicsObj::UpdateObjectInternal (acclient @ 0x00513730):
+///   rootOffset = CPartArray::Update(dt)             // animation
+///   PositionManager::adjust_offset(rootOffset)      // adds correction
+///   frame.origin += rootOffset
+///
+/// In acdream the animation root motion is sourced from
+/// AnimationSequencer.CurrentVelocity (body-local velocity from the
+/// active locomotion cycle). We rotate that by the body's orientation
+/// to get a world-space delta, then add the InterpolationManager's
+/// world-space correction.
+/// </summary>
+public sealed class PositionManager
+{
+    /// <summary>
+    /// Compute the per-frame world-space delta to add to body.Position.
+    /// </summary>
+    /// <param name="dt">Per-frame delta time, seconds.</param>
+    /// <param name="currentBodyPosition">Body's current world-space position.</param>
+    /// <param name="seqVel">
+    ///   Body-local velocity from the active animation cycle
+    ///   (from <c>AnimationSequencer.CurrentVelocity</c>); pass
+    ///   <c>Vector3.Zero</c> if the entity has no sequencer or is on a
+    ///   non-locomotion cycle.
+    /// </param>
+    /// <param name="ori">Body orientation; used to rotate seqVel from body-local to world.</param>
+    /// <param name="interp">The remote's InterpolationManager (for AdjustOffset call).</param>
+    /// <param name="maxSpeed">From <c>MotionInterpreter.GetMaxSpeed()</c> — passed to AdjustOffset for the catch-up clamp.</param>
+    public Vector3 ComputeOffset(
+        double dt,
+        Vector3 currentBodyPosition,
+        Vector3 seqVel,
+        Quaternion ori,
+        InterpolationManager interp,
+        float maxSpeed)
+    {
+        // Step 1: animation root motion (body-local → world).
+        Vector3 rootMotionLocal = seqVel * (float)dt;
+        Vector3 rootMotionWorld = Vector3.Transform(rootMotionLocal, ori);
+
+        // Step 2: interpolation correction (world-space already).
+        Vector3 correction = interp.AdjustOffset(dt, currentBodyPosition, maxSpeed);
+
+        // Step 3: combined delta.
+        return rootMotionWorld + correction;
+    }
+}