Honor cancellation during basket migration

[rohenaz]

Summary

• check task cancellation before each legacy basket drain page
• check cancellation before each output internalization
• prove cancellation stops both loops even when the in-flight dependency ignores cancellation
• prove a moved page followed by an empty page completes with a nonzero moved count

Verification

• focused BasketMigrationTests: 9 passed
• full swift test: 261 passed, 2 skipped

This closes the lock-cancellation gap discovered while reviewing mobile PR #18.