Add animation-follower desc YAML to mirror controller transforms

[earlye]

Summary

• New kind: animation-follower desc YAML annotation causes an SVG element to mirror its controller's transform
• Controller is resolved by inkscape:label or SVG id via the controller: key
• Follower transform is applied on every anim_setAbsFrame (timeline scrub/playback) and immediately when the user drags the controller on the canvas (via new SvgElementTrack::captured signal)
• Uses setCurrentBaseValue per leaf animator — O(1) per leaf, not a full keyframe serialize/deserialize

Usage

<desc>kind: animation-follower
controller: head</desc>

Test plan

• Add kind: animation-follower / controller: <name> desc to a layer in an SVG linked into a Friction project
• Verify the follower moves with the controller when scrubbing the timeline
• Verify the follower moves with the controller when dragging the controller element on the canvas
• Verify just run-debug-followers logs binding resolution and per-frame propagation
• Verify a missing controller name logs a warning and does not crash

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