feat(core+app): Phase 6.4 — per-frame animation playback (breathing/idle cycles)

Phase 6.1-6.3 resolved the right cycle and rendered its first frame
as a static pose. Phase 6.4 actually walks the cycle over time so
creatures, characters, and props animate their idle motion — the
breathing the user noticed was missing after Phase 6.1.

MotionResolver gains GetIdleCycle() returning IdleCycle(Animation,
LowFrame, HighFrame, Framerate). The existing GetIdleFrame() now
shares a private ResolveIdleCycleInternal helper, so the resolution
algorithm (motion-table override, stance/command priority, fallback)
is identical for both entry points and stays in one place.

WorldEntity.MeshRefs becomes a get/set so the per-frame tick can
swap in fresh per-part transforms without rebuilding the entity.
Static decorations never get touched.

GameWindow keeps a Dictionary<entityId, AnimatedEntity> for entities
whose motion table resolved to a multi-frame, non-zero-framerate
cycle. AnimatedEntity caches a per-part template (gfxObjId +
surfaceOverrides + scale) snapshot taken from the hydration pass so
the tick doesn't redo AnimPartChange/TextureChange resolution every
frame — only the per-part transform matrices are recomputed.

OnRender calls TickAnimations(dt) before Draw. The tick advances each
entity's CurrFrame by dt*Framerate, wraps it inside [LowFrame, HighFrame],
samples the corresponding AnimationFrame, and rebuilds the entity's
MeshRefs by composing scale → quaternion rotate → translate per part
in the same order SetupMesh.Flatten uses, then baking the entity's
ObjScale on top in the same PartTransform * scaleMat order as the
hydration path.

160 tests green.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Meshing/MotionResolver.cs

@@ -50,12 +50,65 @@ public static class MotionResolver
     /// — pass that here when present so character outfits/states use the
     /// correct table.
     /// </param>
+    /// <summary>
+    /// Per-entity idle-cycle playback descriptor returned by
+    /// <see cref="GetIdleCycle"/>. Holds the resolved Animation plus the
+    /// frame range and framerate from the MotionTable's AnimData entry.
+    /// Phase 6.4 uses this to drive per-frame playback (so creatures
+    /// breathe instead of being frozen on a single frame).
+    /// </summary>
+    public sealed record IdleCycle(
+        Animation Animation,
+        int LowFrame,
+        int HighFrame,
+        float Framerate);
+
+    /// <summary>
+    /// Same resolution algorithm as <see cref="GetIdleFrame"/> but
+    /// returns the full cycle metadata so the caller can advance frames
+    /// over time. Returns null if any link in the chain is missing.
+    /// </summary>
+    public static IdleCycle? GetIdleCycle(
+        Setup setup,
+        DatCollection dats,
+        uint? motionTableIdOverride = null,
+        ushort? stanceOverride = null,
+        ushort? commandOverride = null)
+    {
+        var resolved = ResolveIdleCycleInternal(setup, dats, motionTableIdOverride, stanceOverride, commandOverride);
+        if (resolved is null) return null;
+        var (anim, ad) = resolved.Value;
+        return new IdleCycle(anim, ad.LowFrame, ad.HighFrame, ad.Framerate);
+    }
+
     public static AnimationFrame? GetIdleFrame(
         Setup setup,
         DatCollection dats,
         uint? motionTableIdOverride = null,
         ushort? stanceOverride = null,
         ushort? commandOverride = null)
+    {
+        var resolved = ResolveIdleCycleInternal(setup, dats, motionTableIdOverride, stanceOverride, commandOverride);
+        if (resolved is null) return null;
+        var (animation, animData) = resolved.Value;
+        int frameIdx = animData.LowFrame;
+        if (frameIdx < 0 || frameIdx >= animation.PartFrames.Count)
+            frameIdx = 0;
+        return animation.PartFrames[frameIdx];
+    }
+
+    /// <summary>
+    /// Shared cycle-resolution path for both <see cref="GetIdleFrame"/>
+    /// and <see cref="GetIdleCycle"/>. Returns the loaded Animation +
+    /// the AnimData that describes its frame range and framerate, or
+    /// null if any link in the chain is missing.
+    /// </summary>
+    private static (Animation, AnimData)? ResolveIdleCycleInternal(
+        Setup setup,
+        DatCollection dats,
+        uint? motionTableIdOverride,
+        ushort? stanceOverride,
+        ushort? commandOverride)
     {
         ArgumentNullException.ThrowIfNull(setup);
         ArgumentNullException.ThrowIfNull(dats);
@@ -127,7 +180,7 @@ public static class MotionResolver
 
         var animData = motionData.Anims[0];
 
-        // Step 3: load the Animation and grab the first frame.
+        // Load the Animation referenced by the cycle.
         uint animId = (uint)animData.AnimId;
         if (animId == 0) return null;
 
@@ -135,12 +188,6 @@ public static class MotionResolver
         if (animation is null) return null;
         if (animation.PartFrames.Count == 0) return null;
 
-        // animData.LowFrame is the start frame index of the cycle within the
-        // animation. Clamp defensively in case it's out of range.
-        int frameIdx = animData.LowFrame;
-        if (frameIdx < 0 || frameIdx >= animation.PartFrames.Count)
-            frameIdx = 0;
-
-        return animation.PartFrames[frameIdx];
+        return (animation, animData);
     }
 }