Full rescan when site coordinates change >1 degree

The planner fast-path (RecomputeForDate) only recalculates altitude profiles
for existing targets. When the site moves significantly (e.g. -37S to 50N),
southern-hemisphere targets would show 0 degrees with no replacement.

Now compares new lat/lon against previous values and triggers a full
ComputeTonightsBestAsync when the difference exceeds 1 degree.

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

Author: sebgod

TODO.md

@@ -3,6 +3,8 @@
 ## Next Up
 
 - [x] Fake filter wheels should have pre-installed filters (realistic filter sets per device ID)
+- [ ] Planner: full rescan when site coordinates change significantly (>1°) instead of fast-path recompute — currently changing lat from -37 to 50 keeps southern-hemisphere targets with 0° altitude
+- [ ] Extract VkImageRenderer UI layout to Abstractions — toolbar, file list, status bar, hit testing are renderer-agnostic; image rendering + texture upload stay Vulkan-specific in Shared
 - [ ] Pinned items in planner should persist to disk (`<CommonDataRoot>/Sessions/Uncommitted.json`) so they survive app restarts while not yet committed to a planned session
 - [ ] Seed focuser `MaxStep` from hardware during ZWO EAF discovery (same `seedQueryParams` pattern as EFW slot count)
 - [ ] Remember last focus position in profile URI after auto-focus (save after every auto-focus attempt, whether successful or not) so the focuser can start near the last known good position on next session

src/TianWen.UI.Gui/Program.cs

@@ -222,13 +222,17 @@ await PlannerActions.ComputeTonightsBestAsync(
                             transform.DateTimeOffset = noon;
                         }
 
+                        // Detect significant site change (>1°) — requires full rescan, not just recompute
+                        var siteChanged = Math.Abs(transform.SiteLatitude - plannerState.SiteLatitude) > 1.0
+                            || Math.Abs(transform.SiteLongitude - plannerState.SiteLongitude) > 1.0;
+
                         plannerState.SiteLatitude = transform.SiteLatitude;
                         plannerState.SiteLongitude = transform.SiteLongitude;
                         plannerState.SiteTimeZone = transform.SiteTimeZone;
 
-                        // Fast path: if we already have targets, just recompute night window + profiles
-                        // Full rescan only needed on first load
-                        if (plannerState.TonightsBest.Count > 0)
+                        // Fast path: if we already have targets and site didn't change significantly,
+                        // just recompute night window + altitude profiles
+                        if (plannerState.TonightsBest.Count > 0 && !siteChanged)
                         {
                             PlannerActions.RecomputeForDate(plannerState, transform);
                         }