games: fix timing drift with cumulative nextTick, 30fps render, 90 pps base

Root cause of unstable speed: nextTick was reset to 'now + speed' each step,
accumulating timing drift. Now uses 'nextTick += speed' with a floor (if
nextTick < now, reset to now) to prevent drift and catch-up spirals.

- Both games: 30 fps render cap (FRAME_MS = 33 ms), nextTick updated
  cumulatively to eliminate drift.
- Snake: base speed 90 px/sec (~100 ms per step at cell=9), turn queue
  unchanged, edge wrap unchanged.
- Tetris: gravity step uses cumulative nextTick, controls unchanged.

src/games/snake.cpp

@@ -228,7 +228,8 @@ static void step() {
     update_speed();
     place_food();
   }
-  nextTick = now_ms() + speed;
+  if (nextTick < now_ms()) nextTick = now_ms();
+  nextTick += speed;
 }
 
 static void draw_text(int x, int y, int scale, unsigned long col, const char* s) {
@@ -407,7 +408,7 @@ int main() {
   render();
 
   int xfd = ConnectionNumber(dpy);
-  const unsigned long FRAME_MS = 1000 / 60;  // 60 fps rendering
+  const unsigned long FRAME_MS = 1000 / 30;  // 30 fps rendering
   unsigned long frameStart = now_ms();
 
   while (running) {
@@ -425,7 +426,8 @@ int main() {
     // Game logic step at its own rate
     if (now >= nextTick) {
       if (state == PLAYING) step();
-      nextTick = now + (state == PLAYING ? speed : 80);
+      if (nextTick < now) nextTick = now;
+      nextTick += (state == PLAYING ? speed : 80);
     }
 
     // Render every frame

src/games/tetris.cpp

@@ -520,7 +520,7 @@ int main() {
   render();
 
   int xfd = ConnectionNumber(dpy);
-  const unsigned long FRAME_MS = 1000 / 60;  // 60 fps rendering
+  const unsigned long FRAME_MS = 1000 / 30;  // 30 fps rendering
   unsigned long frameStart = now_ms();
 
   while (running) {
@@ -538,7 +538,8 @@ int main() {
     // Game logic step at its own rate
     if (now >= nextTick) {
       if (state == PLAYING) step();
-      nextTick = now + (state == PLAYING ? speed : 80);
+      if (nextTick < now) nextTick = now;
+      nextTick += (state == PLAYING ? speed : 80);
     }
 
     // Render every frame