Live stepper robustness: manual steps always snapshot; clearer no-history status

Moved the step-back snapshot out of liveAdvance: a manual forward step now ALWAYS records the
pre-step state (rate-independent, so changing the rate or mixing play+step no longer makes the
back-step jump), while a continuous run records a throttled snapshot (~1/sec). Step-back with no
recorded past (e.g. immediately after a load) now shows a clear status instead of silently
freezing the storms. Forward-then-back reverses weather + storms as before.

Note: a load restores the current storms (v11) but not their past trajectory, so stepping back
BEFORE the loaded moment can only rewind the sky -- reversing loaded storms needs the rewind
history, which isn't persisted (see follow-up).

Build clean; logic/weather suites pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/render/Viewer.cpp

@@ -305,7 +305,9 @@ void Viewer::stepSim() {
     if (liveWorld) {
         // --- Live World: advance the slow clock; geology is frozen --------
         double dtH = (!paused) ? liveRate * GetFrameTime() : 0.0;   // simulated hours this frame
-        liveAdvance(dtH, dtH);           // liveAdvance records step-back snapshots itself
+        if (dtH > 0.0 && (wxUndo.empty() || liveTime - wxUndo.back().t >= liveRate - 1e-9))
+            wxPushSnapshot();            // throttled history during a continuous run (~1 snapshot/sec)
+        liveAdvance(dtH, dtH);
         return;
     }
     if (!paused && !settled) {
@@ -344,17 +346,6 @@ void Viewer::stepSim() {
 // integrated, non-reversible path, so it advances by dtWeather (0 = hold, used for a backward
 // step which still rewinds the deterministic sky: day/night, tides, seasons, moon phases).
 void Viewer::liveAdvance(double dtClock, double dtWeather) {
-    // Record a step-back snapshot of the PRE-advance state at ~one-step cadence, on ANY forward
-    // advance (continuous run or manual step) -- so stepping back reverses weather + storms whether
-    // they were born while stepping or during a continuous run. interval ~ liveRate means roughly
-    // one snapshot per real second regardless of the clock rate (bounded ring, drops oldest).
-    if (dtClock > 0.0) {
-        double interval = std::max(1e-6, liveRate);
-        if (wxUndo.empty() || liveTime - wxUndo.back().t >= interval - 1e-9) {
-            if ((int)wxUndo.size() >= wxUndoMax) wxUndo.erase(wxUndo.begin());
-            wxUndo.push_back(WxFrame{ liveTime, planet.captureWeather() });
-        }
-    }
     liveTime = std::max(0.0, liveTime + dtClock);
     double days        = liveTime / planet.cfg.dayLengthHours;
     double dayOfYear01 = days / planet.cfg.yearLengthDays; dayOfYear01 -= std::floor(dayOfYear01);
@@ -375,10 +366,17 @@ void Viewer::liveAdvance(double dtClock, double dtWeather) {
     rebuildLiveOverlay();
 }
 
-// Step the live clock forward one rate-unit. Auto-pauses (like a video frame-step); liveAdvance
+// Push the current (pre-advance) weather state onto the bounded step-back ring.
-// records the pre-step snapshot so the backward step can restore weather + storms exactly.
+void Viewer::wxPushSnapshot() {
+    if ((int)wxUndo.size() >= wxUndoMax) wxUndo.erase(wxUndo.begin());
+    wxUndo.push_back(WxFrame{ liveTime, planet.captureWeather() });
+}
+
+// Step the live clock forward one rate-unit. Auto-pauses (like a video frame-step); always records
+// the pre-step snapshot first (rate-independent) so the backward step restores weather + storms.
 void Viewer::liveStepForward() {
     paused = true;
+    wxPushSnapshot();
     liveAdvance(liveRate, liveRate);
 }
 
@@ -393,8 +391,10 @@ void Viewer::liveStepBack() {
         liveTime = f.t;
         planet.restoreWeather(f.w);
         liveAdvance(0.0, 0.0);        // recompute the sky/overlay at the restored time (weather held)
+        setStatus("Step back");
     } else {
-        liveAdvance(-liveRate, 0.0);  // no history: deterministic sky rewinds, weather holds
+        liveAdvance(-liveRate, 0.0);  // no recorded past (e.g. right after a load): sky rewinds, weather holds
+        setStatus("Step back (sky only - no earlier weather; play/step forward first)");
     }
 }
 

src/render/Viewer.hpp

@@ -145,6 +145,7 @@ struct Viewer {
     void liveAdvance(double dtClock, double dtWeather);  // advance the Live World clock + fields
     void liveStepForward();              // step the clock forward one rate-unit (snapshots for undo)
     void liveStepBack();                 // step everything back one frame (restores weather/storms)
+    void wxPushSnapshot();               // push the current weather state onto the step-back ring
     Rectangle mapViewRect() const;       // 2D map projection rect after zoom/pan (scissor stays mapRect)
 
     // ---- Input (ViewerInput.cpp) --------------------------------------------

src/render/ViewerInput.cpp

@@ -211,7 +211,7 @@ void Viewer::handleInput() {
     // backward restores the snapshot from the last forward step -> everything (incl. weather +
     // storms) steps back, within the current paused stepping session.
     if (IsKeyPressed(KEY_PERIOD) && liveWorld) { liveStepForward(); setStatus("Step forward"); }
-    if (IsKeyPressed(KEY_COMMA)  && liveWorld) { liveStepBack();    setStatus(wxUndo.empty() ? "Step back (sky only)" : "Step back"); }
+    if (IsKeyPressed(KEY_COMMA)  && liveWorld) { liveStepBack(); }   // liveStepBack sets its own status
     if (IsKeyPressed(KEY_F))     {                            // fast-forward to settled
         if (!settled) {
             while (!settled) stepOnce();