Update todo

todo.md

@@ -4,6 +4,10 @@
 - better error reporting and stack traces in strict mode
 - auto-enable of strict mode depending on compiler optimizaton level
 - check smoothness of spring at high frequency updates
+- have reactive recomputations track sources dynamically
+  - addresses case where sources are used in if-branching guarded by another
+    source
+  - performance implications
 - implement from solid
   - [x] onCleanup > `cleanup()`
   - [x] Index > `indexes()`