fix: clear selections before track extension to prevent stale outlines

[Aman-Cool]

Issue & Fix Summary

Issue

Selection Outline Desynchronization after Track Extension

When a track was selected and the track extension feature was enabled, the track geometry was replaced but the selection outline was not updated. As a result, the outline continued to show the original track length while the actual track extended, causing silent visual desynchronization and misleading selection feedback.

Fix

To prevent outlines from referencing stale geometry, all selections and hover outlines are cleared before extending track geometry.

Changes Made

1. three-manager

   • Added extendCollectionTracks() method that:
     • Calls selectionManager.clearAllSelections()
     • Clears hover outlines via effectsManager.setHoverOutline(null)
     • Then invokes sceneManager.extendCollectionTracks()

2. UI Call Sites

   • Updated Phoenix Menu and dat.GUI to call three.extendCollectionTracks() instead of accessing sceneManager directly.

Result

• Selection outlines always match current track geometry
• No stale or disposed geometry is referenced
• Users must reselect tracks after extension, but visual feedback is always correct

[Aman-Cool]

@EdwardMoyse , Track extension replaces geometry in place, which makes existing selection outlines stale. Clearing selections before extension avoids misleading visuals and references to disposed geometry. Re-selecting tracks is a reasonable tradeoff to keep visual feedback correct and predictable.