fix compute_schedule_filt: scoping bug and reserved word 'stop'

- 'let s' was declared inside if/else blocks (block-scoped),
  making it invisible to 'return s' outside — declare s before the blocks
- 'stop' is a reserved identifier in Espruino (Shelly JS engine),
  rename to t_start/t_stop

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pool.js

@@ -72,17 +72,18 @@ function compute_filtration_time(t) {
 // [ start1, stop1, start2, stop2, ... ]
 
 function compute_schedule_filt(d) {
-  let start = 17 - d/2;
-  let stop = 17 + d/2;
+  let t_start = 17 - d/2;
+  let t_stop = 17 + d/2;
 
-  while (stop >= 24)
-    stop = stop - 24;
+  while (t_stop >= 24)
+    t_stop = t_stop - 24;
 
+  let s;
   if (d < 24) {
-    let s = [ start, stop ];
+    s = [ t_start, t_stop ];
   }
   else {
-    let s = [ start ];
+    s = [ t_start ];
   }
 
   return s;