animtaion blend with bone mask and suport transitions

## Aim while walking

Imagine you character is walking and you decide do Aim.

## blend tree that can pass to transition player from any blend node

we can add Aim animation just to upper body, of course with transition. we need some blend tree. if you worked with Godot animation tree in blend-tree we can define some pipeline of blend animations of nodes that can be on/of or change value

## current blend graph

current blend graph have weird bite-wise bone select for mask create! also it dose not support animation transition.
in official example we play masked part individual with multi animation player, not single animation transition player that resolve animations

## Additional context
it is so good if consider oneShot node and 1d, 2d blend spaces like Godot. also one point Godot has lake, some nodes like blend spaces can have input values for coordinate. imagine we have one single state about direction, but different movement like walk, run or sneak! in godot we need set direction individual for each blend space because the blend spaces dose not input node and coordinates are all internal, it is so good it can come from some input node like vector2d

No files, tests, or entry points are named. Start by locating Bevy's animation tree and blend graph implementation, then compare its bone-mask and transition behavior with the described walking-to-aim case. Done should cover masked upper-body blending with transitions and clarify whether OneShot and 1D/2D blend-space inputs are included.

Written by the indexing model from the issue text.