feat: Publisher interface and adapter event types

[klaidliadon]

Adds an observability layer for adapters. A runnable.Publisher interface (single Publish(any) method) is installed onto the runnable's ctx via runnable.WithPublisher; adapters call runnable.Publish(ctx, event) to emit typed events. StatusStore becomes a Publisher and Status.Restarts is back — counted from RetryEvent instead of the deprecated WithRetry onStart side-channel.

Stacked on #4 — review against feat/drain-and-ticker, not master.

Closes marino39's #2 quick-glance item (StatusStore refactor so adapters can store stuff).

---

Commits

• 01eed91 — Publisher interface + Publishers fanout + WithPublisher Option + ctx helpers (PublisherFrom, Publish). Event types (RetryEvent, DrainStartedEvent, DrainTimedOutEvent, PanicRecoveredEvent).
• 94d8068 — Wire publish calls into Retry / Draining / Recovering. Recovering drops its PanicHandler argument (single way to handle panics: subscribe via Publisher).
• 8700487 — WithStatus registers a tagged statusPublisher; Status.Restarts increments from RetryEvent. Integration test (in runnable_test to avoid an import cycle) verifies WithStatus + adapters.Retry end-to-end.
• 5616225 — README "Observability via Publisher" section + updated migration table.

Shape

type log struct{}

func (log) Publish(event any) {
    switch ev := event.(type) {
    case runnable.RetryEvent:
        slog.Warn("retry", "attempt", ev.Attempt, "err", ev.Err)
    case runnable.PanicRecoveredEvent:
        slog.Error("panic", "err", ev.Recovered)
    }
}

r := runnable.New(work,
    runnable.WithStatus("recon", store), // store sees RetryEvent → Restarts++
    runnable.WithPublisher(log{}),       // log sees the same events
    runnable.WithAdapters(
        adapters.Retry(3, time.Minute),
        adapters.Recovering(),
    ),
)

Test plan

• go test -race -count=1 ./... — passes.
• Vacuous-test check: commented out the r.publisher = mergePublisher(...) wiring in WithStatus.apply and confirmed TestStatusRestartsCountedFromRetryAdapter fails (expected 2, got 0) before restoring.
• golangci-lint run ./... — clean.

Notes

• Publisher signature is Publish(event any) — single method, no source/ID parameter. WithStatus tags events with the runnable ID via an internal statusPublisher wrapper.
• Multi-publisher composition: WithPublisher (and WithStatus) stack additively via a Publishers fanout type. Order doesn't matter.
• Synchronous dispatch: Publisher.Publish runs on the goroutine that emits. Documented in README. Subscribers needing async dispatch should buffer internally.
• Breaking change carried in this stack: adapters.Recovering() no longer takes a PanicHandler. Migration is one extra Option (WithPublisher) — documented.

[marino39]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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[marino39]

LGTM - however you need to rebase