[retry-core] Offer AbortSignal-aware retry and backoff cancellation

[kang-heewon]

Priority: P1

Problem

Retry APIs have no AbortSignal and RetryEngine waits for backoff unconditionally. A cancelled request can remain queued and execute more attempts after its caller has ended.

Evidence

packages/retry-core/src/libs/RetryTemplate.ts and RetryOrchestrator.ts expose no signal; RetryEngine.ts:30-92 only stops through hooks and waits through the backoff policy.

Desired outcome

Retry work stops promptly and deterministically on caller cancellation.

Implementation path

1. Add optional signal to template, orchestrator, engine, and decorator paths where applicable.
2. Check cancellation before attempts and during backoff with an abortable wait.
3. Emit a stable RetryAbortedProblem with safe context.
4. Test before-first-attempt, during-backoff, listener cleanup, and no-further-attempt behavior.

Acceptance criteria

• Cancellation prevents subsequent attempts.
• Backoff does not outlive an aborted signal.
• Existing retry behavior is unchanged when no signal is supplied.

Validation

pnpm --filter @croco/retry-core test

Scope boundaries

Does not redesign retry classification or backoff algorithms.