[TRACKER] Full body & generic inverse kinematics for Skeleton3D in Godot Engine

Since the inverse kinematics work is a sprawling open problem. I've made a tracker issue.

In master:
- [x] https://github.com/godotengine/godot/pull/110120
- [x] https://github.com/godotengine/godot/pull/111815
- [x] https://github.com/godotengine/godot/pull/112524
- [x] https://github.com/godotengine/godot/pull/110336
- [x] https://github.com/godotengine/godot/pull/111367
- [x] https://github.com/godotengine/godot/pull/111184
- [x] https://github.com/godotengine/godot/pull/110506
- [x] https://github.com/godotengine/godot/pull/110376
- [ ] https://github.com/godotengine/godot/pull/109009

TBA:
- [ ] Prepare release notes materials
- [ ] #112308
- [ ] https://github.com/godotengine/godot-demo-projects/pull/1271
- [ ] #113640
- [x] https://github.com/godotengine/godot-proposals/issues/6039
- [x] https://github.com/godotengine/godot/pull/90316

Treat this as a tracker rather than a self-contained task. Review the unchecked items, especially PR 109009 and issues #112308 and #113640, along with the completed inverse-kinematics links, to understand the remaining scope. Done means the outstanding checklist items and release-note materials are resolved.

Written by the indexing model from the issue text.