chore: add changeset for critical scheduler fixes

Author: shtse8

.changeset/critical-scheduler-fixes.md

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+---
+"@sylphx/zen": patch
+---
+
+Critical bug fixes for scheduler and subscribe API
+
+**Fixed Bugs:**
+
+1. **subscribe() undefined handling** - Fixed bug where signals with undefined initial values would skip first 2 updates. Now uses hasValue flag instead of undefined check.
+
+2. **Dead work elimination** - Added guard to queueBatchedNotification to skip queue when no legacy effect listeners exist. Eliminates unnecessary allocations in effect-based subscribe apps.
+
+3. **Error recovery** - Clear all queue flags in finally block to prevent stuck state when effects/computeds throw errors. Ensures scheduler can recover from errors correctly.
+
+**Impact:**
+- Fewer allocations (no pending notifications without legacy listeners)
+- Safer error handling (flags always cleared on errors)
+- Correct undefined handling (no silent bugs with undefined values)
+
+All 48 tests passing.