test(editor): add unit tests for AudioUltra Segment (parity-13)

Add libs/editor/src/lib/AudioUltra/Regions/__tests__/Segment.test.ts with 38
tests covering Segment constructor (validation, defaults, options), update,
setVisibility, bringToFront, getters (options, width, timelineHeight/Placement),
render (visible/viewport), handleUpdateEnd, handleSelected, handleHighlighted,
setColor, setLocked, updateColor, updatePosition, scrollToRegion,
convertToRegion/convertToSegment, remove, destroy, toJSON, and switchCursor.
Segment.ts at coverage parity (target 95.08%).

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: yyassi-heartex