content: regenerate Season 1 with a gentle onboarding difficulty curve

Generator: raise the early bot-win-rate floor to ~0.97 (near-guaranteed first
stages), widen the early move-buffer margin, and delay score-chase/line-sprint
objectives until stage 9/13 so the opening stretch is pure clear-a-gem. Result:
stage 1 now 16 moves (was 11), stages 1-9 all simple gems at 90-99% bot win.
Preserves theme.bgm=season_001; manifest SHA regenerated. Settings tag -> build 4.

tool/stage_generator/generate.dart

@@ -106,8 +106,10 @@ ${reportRows.join('\n')}
 /// weaker than the bot, so the floor stays high — and early stages are
 /// allowed to be a guaranteed win (onboarding).
 (double, double) _band(double t) => (
-      0.78 - 0.30 * t,
-      t < 0.4 ? 1.0 : 1.0 - 0.10 * (t - 0.4) / 0.6,
+      // Near-guaranteed onboarding (the bot easily clears the first stages, so
+      // a human does too), ramping to a genuine challenge by the season's end.
+      0.97 - 0.50 * t,
+      t < 0.5 ? 1.0 : 1.0 - 0.12 * (t - 0.5) / 0.5,
     );
 
 (StageConfig, double, List<int>)? _buildCandidate(
@@ -145,7 +147,9 @@ ${reportRows.join('\n')}
 
   movesToWin.sort();
   final median = movesToWin[movesToWin.length ~/ 2];
-  final margin = 0.45 - 0.25 * t;
+  // Generous early move buffer over what the bot needs (humans place less
+  // optimally and the piece RNG won't always cooperate), tightening later.
+  final margin = 1.0 - 0.65 * t;
   var moveLimit = (median * (1 + margin)).ceil().clamp(6, 60);
 
   // Validate at the real budget; nudge the budget up/down until the win
@@ -248,11 +252,12 @@ Objective _sampleObjective(
     int index, double t, List<PresetCell> preset, SeededRng rng) {
   final gemCount =
       preset.where((c) => c.type == CellType.gem).length;
-  // Variety beats: every 5th stage is a score chase, every 7th a line sprint.
-  if (index % 7 == 6) {
+  // Keep the opening stretch pure "clear a gem" onboarding; introduce the
+  // score-chase and line-sprint variety only once the player has settled in.
+  if (index >= 12 && index % 7 == 6) {
     return Objective.clearLines(3 + (t * 7).round() + rng.nextInt(2));
   }
-  if (index % 5 == 4) {
+  if (index >= 8 && index % 5 == 4) {
     return Objective.reachScore(600 + (t * 2400).round() + rng.nextInt(200));
   }
   return Objective.clearGems(gemCount);