Loop optimization enhacements

[creightonlinza]

Summary

Improves loop optimization (anchor/forced-jump selection) to prefer better real-world outcomes, removes the old loop-extension toggle path, and adds an optional forced-anchor highlight in the viz.

Key Changes

1. Reworked anchor selection in web/src/engine/graph.ts with lookahead/tiered candidate logic to better balance late placement and ability to return earlier in track.
2. Improved forced-neighbor choice at playback time in web/src/engine/selection.ts using downstream reachability.
3. Removed legacy loop-extension toggle support:
   • UI row removed
   • addLastEdge removed from config/type surface
   • lb tuning param removed from web/cast tuning handling
4. Added optional UI setting: Highlight forced anchor jump.
5. Persisted that highlight preference via localStorage (fj-highlight-anchor-branch).

Testing in progress