Fix stale altitude charts after site change

ComputeTonightsBestAsync did not invalidate the cached astrom grid,
so altitude profiles were computed using the old site's precomputed
astrometric data. Now clears CachedAstromGrid before recomputing
profiles, matching what RecomputeForDate already does.

Also add TODOs for VkImageRenderer abstraction extraction.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: sebgod

src/TianWen.UI.Abstractions/PlannerActions.cs

@@ -140,6 +140,8 @@ void Report(string msg)
         // Compute fine altitude profiles and cross-index aliases for the top targets
         Report("Computing altitude profiles...");
 
+        // Invalidate cached grid (site or date may have changed)
+        state.CachedAstromGrid = null;
         var (gridAstroms, gridTimes) = EnsureAstromGrid(state, transform);
 
         state.AltitudeProfiles.Clear();