Fix: step-back now reverses run-born storms (record undo history continuously)

The step-back undo history was cleared on every continuous-run frame, so weather systems
(storms/hurricanes) created during a normal run had no recorded past. Stepping back then only
rewound the deterministic sky and left the storm frozen at its current spot, resuming motion
only on a forward step.

liveAdvance() now records a snapshot of the pre-advance weather state at ~one-step cadence on
ANY forward advance (continuous run or manual '.'), not just manual steps -- the interval
scales with liveRate, so it's ~one snapshot per real second at any clock rate, in a bounded
ring. liveStepBack() searches the ring for the most recent snapshot before the current time and
restores it (clock + humidity/cloud/rain + storms + RNG), so storms reverse regardless of when
they were born. The clear-on-run was removed; entering Live World still resets the ring.

All five suites pass; GUI build clean. Docs updated.

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

CLAUDE.md

@@ -425,10 +425,12 @@ Working and verified (logic tested headless):
   dtWeather)`; **`.`** steps forward and **`,`** back by `liveRate` hours (both auto-pause, like a
   video frame-step). Weather is an integrated path (not analytically reversible), so a forward step
   snapshots the full state — `Planet::captureWeather()`/`restoreWeather()` (humidity/cloud/rain/
-  storms/RNG) into a bounded `wxUndo` history — and **`,` restores the previous snapshot**, so the
-  step really reverses *everything* (clouds, rain, storms) plus the deterministic sky. A continuous
-  run (unpause) clears the undo history (so `,` then falls back to a sky-only rewind). `S` in Live
-  World aliases the forward step.
+  storms/RNG) into a bounded `wxUndo` ring — and **`,` restores the previous snapshot**, so the
+  step really reverses *everything* (clouds, rain, **moving storms**) plus the deterministic sky.
+  `liveAdvance` records a snapshot at ~one-step cadence on **any** forward advance (continuous run or
+  manual step), so storms born during a continuous run also rewind (an earlier version cleared the
+  history on run, which left run-born storms frozen on step-back); `,` searches the ring by time, the
+  ring drops oldest past `wxUndoMax`. `S` in Live World aliases the forward step.
 - Mouse hover (in either view) shows per-cell info. Clicking a tile opens a
   right-side detail panel: tile info header + the tile's subgrid drawn as a
   flat hoverable grid of subtiles (neighbor-owned subtiles dimmed). A high-res

docs/design-notes.md

@@ -223,9 +223,11 @@ Three viewer-only controls over the Live World sim:
   overlay). `.`/`,` step ±`liveRate` hours and **auto-pause** (frame-step). Weather is integrated
   and not analytically reversible, so a forward step snapshots the full weather state
   (`Planet::captureWeather`/`restoreWeather` — humidity/cloud/rain/storms/RNG) into a bounded
-  `wxUndo` ring; **`,` restores the previous snapshot**, reversing clouds/rain/storms exactly as well
-  as the deterministic sky. A continuous run clears `wxUndo` (then `,` rewinds the sky only). The
-  snapshot also reseeds the storm RNG so re-stepping forward replays deterministically.
+  `wxUndo` ring; **`,` restores the most recent snapshot before now**, reversing clouds/rain/storms
+  exactly as well as the deterministic sky. `liveAdvance` records a snapshot at ~one-step cadence on
+  *any* forward advance — continuous run or manual step — so storms born during a run also rewind
+  (the ring is bounded, ~one snapshot per real second since the interval scales with `liveRate`). The
+  restored snapshot includes the storm RNG, so re-stepping forward replays deterministically.
 
 ## Headless testing
 

src/render/Viewer.cpp

@@ -304,8 +304,7 @@ 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
-        if (dtH > 0.0) wxUndo.clear();   // a continuous run invalidates the manual step-back history
-        liveAdvance(dtH, dtH);
+        liveAdvance(dtH, dtH);           // liveAdvance records step-back snapshots itself
         return;
     }
     if (!paused && !settled) {
@@ -344,6 +343,17 @@ 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);
@@ -364,19 +374,19 @@ void Viewer::liveAdvance(double dtClock, double dtWeather) {
     rebuildLiveOverlay();
 }
 
-// Step the live clock forward one rate-unit. Auto-pauses (like a video frame-step) and snapshots
-// the pre-step state so the backward step can restore weather + storms exactly.
+// Step the live clock forward one rate-unit. Auto-pauses (like a video frame-step); liveAdvance
+// records the pre-step snapshot so the backward step can restore weather + storms exactly.
 void Viewer::liveStepForward() {
     paused = true;
-    if ((int)wxUndo.size() >= wxUndoMax) wxUndo.erase(wxUndo.begin());
-    wxUndo.push_back(WxFrame{ liveTime, planet.captureWeather() });
     liveAdvance(liveRate, liveRate);
 }
 
-// Step everything back one frame: restore the last snapshot (clock + weather + storms) if we have
-// one; otherwise fall back to rewinding the deterministic sky only (day/night, tides, seasons).
+// Step everything back: restore the most recent snapshot strictly before the current time (clock +
+// weather + storms) -- so storms reverse whether they were born while stepping or during a run. If
+// the history is exhausted, fall back to rewinding the deterministic sky only.
 void Viewer::liveStepBack() {
     paused = true;
+    while (!wxUndo.empty() && wxUndo.back().t >= liveTime - 1e-6) wxUndo.pop_back();  // drop any future frames
     if (!wxUndo.empty()) {
         WxFrame f = wxUndo.back(); wxUndo.pop_back();
         liveTime = f.t;

src/render/ViewerInput.cpp

@@ -172,7 +172,7 @@ void Viewer::handleInput() {
     if (IsKeyPressed(KEY_W) && settled) {           // enter / leave Live World (slow real-time clock)
         liveWorld = !liveWorld;
         if (liveWorld) {
-            phase3Prompt = false; paused = false;
+            phase3Prompt = false; paused = false; wxUndo.clear();
             refreshView();                          // fresh base colours; overlay builds in stepSim
             setStatus("Live World started");
         } else {