feat(animation): attach external animation files to a model

Animation-only FBX files (no mesh, clips only) could not be used:
clips are keyed by node index into the parsed model's node table, so
there was no path to get them onto a different model's skeleton.

Add AssetManager::AttachAnimationClips: parses the container with the
CPU-only path (no GPU upload -- empty meshes cannot build a Model),
remaps each clip's channels to the target model's node indices by
bone name, and appends to its clip list. Channels whose bone name is
absent are skipped and counted; clips with zero matching channels are
dropped. Same-source attach is idempotent via a new
AnimationClip::sourcePath marker; name clashes get the file stem
prefixed.

Inspector Animation block gains an attach slot (UE Details semantics)
listing model-container files plus a bullet list of attached sources;
already-attached paths are replayed idempotently when the entity is
selected, so clips survive restarts. The paths persist in the scene
(Animation.Attached); the clips themselves stay runtime state.

Limitations (documented, by design): same-skeleton retargeting only
-- bone names must match between source and target (same DCC export
or Mixamo-style per-animation downloads); cross-skeleton retargeting
is out of scope.

Engine/src/Assets/AssetManager.cpp

@@ -9,6 +9,8 @@
 #include <yaml-cpp/yaml.h>
 
 #include <fstream>
+#include <filesystem>
+#include <unordered_map>
 
 namespace Engine::Assets {
 
@@ -124,6 +126,93 @@ namespace Engine::Assets {
             });
     }
 
+    AssetManager::AnimAttachResult AssetManager::AttachAnimationClips(
+        ModelRef target, const std::string& animPath)
+    {
+        AnimAttachResult res;
+        Model* tm = GetModel(target);
+        if (tm == nullptr) {
+            return res;
+        }
+        // 幂等：该源文件已并入过（自愈重放/重复 attach 直接返回）
+        for (const auto& c : tm->GetClips()) {
+            if (c.sourcePath == animPath) {
+                return res;
+            }
+        }
+        // 纯 CPU 解析：动画容器只要 nodes + clips，不建 GPU 资产
+        //（meshes 空的文件过不了 Model 构建，也不需要构建）
+        ParsedModel src = Model::ParseFromDisk(animPath);
+        if (src.clips.empty()) {
+            ENGINE_WARN("AttachAnimationClips: no clips in '{}'", animPath);
+            return res;
+        }
+
+        // 目标模型 骨骼节点名 → nodeIdx 重映射表
+        std::unordered_map<std::string, u32> nameToIdx;
+        nameToIdx.reserve(tm->GetNodes().size());
+        for (u32 i = 0; i < tm->GetNodes().size(); ++i) {
+            nameToIdx.emplace(tm->GetNodes()[i].name, i);
+        }
+
+        const std::string stem =
+            std::filesystem::path(animPath).stem().string();
+        std::vector<AnimationClip> extra;
+        extra.reserve(src.clips.size());
+        for (auto& clip : src.clips) {
+            AnimationClip out;
+            out.name = clip.name;
+            out.duration = clip.duration;
+            out.tps = clip.tps;
+            out.sourcePath = animPath;
+            // 同名冲突（内嵌 clip 或其它源）：源文件名做前缀保持可辨识
+            for (const auto& c : tm->GetClips()) {
+                if (c.name == out.name) {
+                    out.name = stem + "_" + out.name;
+                    break;
+                }
+            }
+            for (auto& ch : clip.channels) {
+                if (ch.positions.empty() && ch.rotations.empty() &&
+                    ch.scales.empty()) {
+                    continue; // 空轨道无用
+                }
+                if (ch.nodeIdx >= src.nodes.size()) {
+                    ++res.channelsSkipped;
+                    continue;
+                }
+                const auto it = nameToIdx.find(src.nodes[ch.nodeIdx].name);
+                if (it == nameToIdx.end()) {
+                    ++res.channelsSkipped; // 骨骼名不同源
+                    continue;
+                }
+                AnimChannel remapped;
+                remapped.nodeIdx = it->second;
+                remapped.positions = std::move(ch.positions);
+                remapped.rotations = std::move(ch.rotations);
+                remapped.scales = std::move(ch.scales);
+                out.channels.push_back(std::move(remapped));
+                ++res.channelsMatched;
+            }
+            // 全部轨道对不上 = 名字空间完全不同源，clip 不入表
+            if (out.channels.empty()) {
+                ENGINE_WARN("AttachAnimationClips: clip '{}' has no matching "
+                            "bones in target (skipped)", clip.name);
+                continue;
+            }
+            extra.push_back(std::move(out));
+            ++res.clipsAdded;
+        }
+        if (!extra.empty()) {
+            tm->AppendClips(std::move(extra));
+        }
+        ENGINE_INFO("AttachAnimationClips: '{}' -> {} clips "
+                    "({} channels matched, {} skipped)",
+                    animPath, res.clipsAdded, res.channelsMatched,
+                    res.channelsSkipped);
+        return res;
+    }
+
     ModelRef AssetManager::CreatePlane(f32 halfSize, u8 r, u8 g, u8 b)
     {
         // 程序化资产复用同一缓存：键 = 伪路径 + 参数