feat(vessel): add Handle.OnTerminate lifecycle hook

[lIang70]

Summary

Adds a synchronous completion-hook API on `vessel.Handle` so multiple consumers can observe run termination with deterministic ordering — no more racy `<-h.Done()` watcher goroutines per consumer.

Why

PR #62 surfaced a real ordering gap on `/v1/vessels/{id}/call`: `/call` returned when `h.Wait` resolved, but `runRegistry`'s background goroutine watching `h.Done()` had no ordering guarantee against `Wait`. Result: `/metrics` and `/v1/runs` could miss a just-completed run, breaking the scrape contract.

The architectural fix is to give `Handle` a proper lifecycle-event hook API instead of patching one consumer at a time.

Contract

• Hooks fire synchronously, in registration order, BEFORE `Wait` returns and BEFORE `Done` is closed.
• Any consumer of `Wait`/`Done` is guaranteed to see a state where every hook has executed.
• Hooks must be fast and non-blocking — they delay every `Wait` caller.
• Hooks may panic; isolated per-hook via `recover` so one buggy hook can't block siblings.
• Late registration is safe: registering after termination invokes `fn` synchronously with the cached terminal state.

Scope

• `vessel/handle.go`: `Handle.OnTerminate(fn)` + `hookMu`/`onTerminate`/`terminated` fields; `deliver` runs hooks under panic isolation before `close(done)`.
• `vessel/handle_test.go`: 8 tests covering ordering, fan-out, panic isolation, late-register fast path, nil-fn no-op, concurrent registration race.

Backwards compatible — pure addition.

Follow-up

PR #62 will pick up `vessel v0.1.0-rc.2` (this PR + tag) and migrate `runRegistry` off its own goroutine, then revert the test-side polling hack added as a stopgap.

Test plan

• `go test -race -count=10 -run TestHandle ./vessel/` green
• `go test -race -count=1 ./vessel/...` green
• CI green

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