Make `SpecularCoverage.get_topk_coverage` return transmitter IDs and unify the coverage output type with `calculate_coverage`

[vinay0000]

Context

SpecularCoverage.calculate_coverage returns List[Tuple[DiscreteCoverageTP, List[float]]], one tuple per GNSS transmitter. get_topk_coverage returns the same tuple type but one entry per rank (best-first by RCG, selected per time step). Because the selected transmitter varies per time step, get_topk_coverage currently drops transmitter identity entirely — unlike its trajectory-domain twin specular.get_topk_trajectories, which returns selected_ids_by_time.

To recover identity, Mission.execute_gnssr_coverage_calculator (added for the topk feature) re-ranks the same RCGs through get_topk_trajectories, feeding it a throwaway placeholder PositionSeries purely as a vehicle. This works (both functions use identical ranking logic), but it's a workaround that couples coverage to the trajectory API and is non-obvious.

Proposal

1. Have get_topk_coverage accept ids and return the selected transmitter id per (time, rank) directly — mirroring get_topk_trajectories.
2. Replace the ad-hoc (DiscreteCoverageTP, List[float]) tuple with a single, shared return type (e.g. a small dataclass carrying coverage, rcg, and optionally transmitter_id(s)) used by both calculate_coverage and get_topk_coverage, so consumers handle one consistent structure.
3. Simplify Mission.execute_gnssr_coverage_calculator to use the new IDs directly and drop the placeholder-PositionSeries workaround.

Acceptance criteria

• get_topk_coverage returns, or makes available, the transmitter id behind each rank at each time step.
• calculate_coverage and get_topk_coverage share one documented return type.
• The placeholder-PositionSeries hack in mission.py is removed.
• Existing callers / the dshield-cygnss-demo example and dshield_format_converter are updated to the new type.

Related: orbitpy/coveragecalculator.py (SpecularCoverage), orbitpy/specular.py (get_topk_trajectories), orbitpy/mission.py (execute_gnssr_coverage_calculator).