feat: multi-drone marker support — make markers selected-drone-aware

[MikePehel]

Problem

The marker system introduced in PR #47 is currently hardcoded to vehicles[0] / sources[0]. In multi-drone replay (PR #50, issue #49), markers are always placed on and associated with the first drone regardless of which drone is selected.

Affected areas:

• marker_positions, marker_times — always reference vehicles[0]
• Bookmark seek — always seeks sources[0]
• System marker resolution — always resolves positions from vehicles[0]'s trail
• [/] marker cycling — jumps through all markers regardless of which drone owns them

Proposed Solution

• Make marker arrays per-vehicle or tag each marker with a vehicle index
• marker_positions and marker_times should reference vehicles[selected] / sources[selected]
• System marker resolution should use the selected drone's origin and trail data
• [/] cycling should only jump to the selected drone's markers, not all drones' markers
• Shift+[/] (fly to marker) should track the correct drone's position

Dependencies

• Depends on PR feat: ULog replay enhancements — frame stepping, persistent trails, markers, and seek fix #47 merging (marker system must exist first)
• Should be compatible with PR feat: multi-drone swarm replay with deconfliction and correlation analysis #50 (multi-drone replay, shared NED origins)
• System markers in formation mode need to account for the shared origin (ref_lat_rad, ref_lon_rad, min_alt)

Related

• PR feat: ULog replay enhancements — frame stepping, persistent trails, markers, and seek fix #47 — marker system implementation
• PR feat: multi-drone swarm replay with deconfliction and correlation analysis #50 — multi-drone replay with deconfliction
• Issue Multi-Log Replay with Ghost Mode and Temporal Alignment #49 — multi-log replay feature request
• Issue Enhance ULog replay with frame stepping, persistent trails, and free camera #41 — original replay enhancement request

[MikePehel]

Satisfied by commit af9c992 (feat: multi-drone marker support with per-drone storage and global cycling). Markers are now per-drone, [/] cycles the selected drone's markers, Ctrl+[/] cycles globally across drones, system markers are initialized per-drone, and fly-to-marker tracks the correct drone's position.