Add BoneAttachment3D node and runtime follow pass

[TiernanDeFranco]

Motivation

• Add a lightweight Node3D-derived attachment node that follows a specific bone on a Skeleton3D so child nodes can inherit final solved bone transforms without mutating skeleton pose.

Description

• Add BoneAttachment3D node type with fields and APIs: base: Node3D, skeleton: Option<NodeID>, bone_index: BoneIndex, enabled: bool, plus setters/getters and Default in perro_nodes (node_3d/bone_attachment_3d.rs).
• Register and export BoneAttachment3D in the 3D node module and node registry so it participates as a Node3D descendant (node_3d/mod.rs, node_registry.rs).
• Extend scene field model and scene loader to parse/apply skeleton, bone_index (alias bone), and enabled for BoneAttachment3D (perro_scene::node_fields.rs, scene loader prepare code).
• Add runtime pass update_bone_attachments that collects enabled attachments, validates skeleton target/type and bone index bounds, copies final bone transform (read-only) into the attachment Node3D transform, marks global transform dirty, and leaves TODO for local offsets; wire the pass to run after internal updates (runtime/internal_updates.rs, runtime.rs).
• Add unit tests covering creation/setters, binding to Skeleton3D, invalid index and disabled safety, and runtime pass transform update (perro_runtime tests).

Testing

• Ran cargo fmt which completed successfully.
• Ran cargo test -p perro_runtime bone_attachment_3d -- --nocapture but the test run was blocked during build by a missing system dependency (alsa.pc) from alsa-sys, so unit tests could not be fully executed in this environment; compilation progressed to dependent crates before failing on the system pkg-config error.
• The new unit tests were added and compile-time issues observed earlier (missing import) were fixed in this rollout prior to the system-level failure.

---

Codex Task