fix: Complete async state machine support for await in loops

- Fix promise pointer clearing after await completion to prevent
  reusing stale promises in subsequent loop iterations
- Add phi node result capturing for proper loop variable persistence
  across state transitions
- Fix multi-block capture analysis to handle loops without await points
- Extend mutable local capturing for multi-block functions

The key fixes:
1. Clear promise pointer in ready block (offset to null) so next
   loop iteration creates a new promise
2. Capture all phi node results as they represent loop variables
3. Don't early-return from analyze_captures for multi-block functions
4. Capture mutable locals for all multi-block functions, not just
   those with await points

Tests now passing:
- test_execute_async_with_await_in_loop: sum_doubled(5) = 30
- test_execute_async_count_up: count_up(n) returns n correctly

Author: darmie