Implement multi-track timeline visualization for different event types

[Copilot]

The timeline visualization hardcoded trackCount = 1, rendering all event types on a single horizontal lane. This replaces it with dynamic track allocation so each TimelineEventType (series, movie, exploration, etc.) gets its own lane.

Types (src/modules/types.ts)

• Added TrackConfig interface (type, label, index)
• Added track-change event to TimelineEvents map

Track system (src/modules/timeline-viz.ts)

• buildTracks() derives tracks from unique event types in filtered data — empty tracks are never rendered
• getEventTrackY() computes vertical position: trackIndex * (trackHeight + trackSpacing) + trackHeight / 2
• Updated all SVG marker/label/indicator cy/y attrs and canvas rendering to use track-aware coordinates
• renderTrackLanes() draws alternating background rects per track
• Track labels rendered on Y-axis via renderAxes()
• Emits track-change when track configuration changes

// Before
const trackCount = 1 // TODO: Multiple tracks for different event types
yScale.domain([0, trackCount])

// After
const newTracks = buildTracks(filteredEvents) // one track per unique event type
tracks = newTracks
yScale.domain([0, Math.max(1, tracks.length)])

Tests (test/timeline-viz.test.ts)

• 5 new tests covering track-change emission, data updates, multi/single-type rendering, and empty events