Extend od_observation_t with observer_idx so each observation can
reference a different ground station. Config now holds an array of
observers instead of a single pointer. The existing single-observer
tle_from_topocentric() is unchanged (sets observer_idx=0 for all obs).
New overload: tle_from_topocentric(topo[], ts[], observer[], int4[], ...)
accepts parallel observer_ids array indexing into the observers array.
PG function overloading resolves by argument types.
Tests 9-11: two-station fit converges, single-station via multi-observer
API matches, out-of-range observer_id raises error.
Ryan Malloy
59fd8ba743