contrib: add jitter to tx-status retry logic

[Elizabethxxx]

Summary

Adds randomized jitter to the polling delay used by waitForTransaction (src/tx-status.ts), which currently sleeps a fixed intervalMs between every poll and can cause synchronized retry spikes across many consumer instances (e.g. a fleet of workers all submitting transactions right after a deploy, then polling in lockstep).

closes #280

What's included

• computeJitteredDelay(intervalMs, options) — "full jitter": each delay is drawn uniformly from [0, intervalMs * (1 + jitterFactor)], then capped at a configurable maxDelayMs (defaults to intervalMs * 10), so a large jitterFactor can never produce an unbounded wait. random is injectable for deterministic tests.
• waitForTransactionJittered(reader, hash, options) — a drop-in replacement for waitForTransaction with the same TxStatus/TxStatusReader types (re-exported from src/tx-status.ts) and the same TransactionTimeoutError behavior, except the sleep between polls comes from computeJitteredDelay. The timeout deadline check accounts for the actual jittered delay used, so a run of unlucky (large) delays still respects timeoutMs.
• 13 tests: computeJitteredDelay produces a real distribution (not one fixed value) across 50 draws, stays within its computed range across 200 draws, is exactly deterministic given an injected random, respects maxDelayMs even with an extreme jitterFactor, defaults maxDelayMs to 10x intervalMs, and rejects negative inputs; waitForTransactionJittered resolves success/failure correctly, times out correctly under worst-case (maximum) jitter, propagates reader errors, uses varying delays across a multi-poll run rather than a fixed interval, and never exceeds a configured maxDelayMs.
• Code comments explain the full-jitter approach and cite the reasoning it follows (AWS Architecture Blog's "Exponential Backoff And Jitter" — the same principle applies to a fixed-interval poll, not just exponential backoff).

Placement note

Submitted under contrib/examples/issue-280-tx-status-retry-jitter/ per the contributor scope rule (contributor PRs may only touch contrib/) as a drop-in-compatible companion to src/tx-status.ts rather than editing it directly.

Test plan

• npx vitest run contrib/examples/issue-280-tx-status-retry-jitter — 13/13 passing
• Standalone strict typecheck of the new files passes with no errors

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