Generalize threshold worker task settlement

[zjma]

Summary

• replace the fetch-specific helper with generic settleUntilThreshold(tasks, threshold) orchestration
• return one ordered fulfilled | rejected | discarded result for every input task
• make cancellation explicitly cooperative/best-effort and abort unfinished tasks after the fulfillment threshold
• move HTTP deadlines into the worker tasks through postWithTimeout; the generic runner has no timeout policy
• migrate IBE basic, IBE custom, and threshold VRF so each opaque task owns fetch, decrypt, parse, and cryptographic validation
• preserve quorum-failure diagnostics while returning immediately once enough tasks fulfill

Closes #156

Validation

• pnpm --dir ts-sdk typecheck
• focused TypeScript tests: 40 passed
• pnpm --dir scenarios test-ibe-aptos-basic
• pnpm --dir scenarios test-ibe-aptos-custom
• pnpm --dir scenarios test-threshold-vrf-derive-flow
• git diff --check