Add src/app/events.go: a sealed Event interface with three concrete
types (JobChanged, RunRecorded, SchedulerStateChanged), an Observer
interface plus ObserverFunc adapter, and Subscribe/emit on the Service.
This replaces the scheduler's single onChange callback with typed
events the UI can exhaustively type-switch over.
Dispatch is serialized by a dedicated dispatchMu (separate from the
state lock): observers never run concurrently, emit must be called
without holding s.mu so observers can read Service state, and observers
must not re-enter an emitting method. emit is wired to mutating ops in
T3.3. Adds tests for ordered multi-observer delivery, empty-observer
no-op, and observer-reads-state-without-deadlock.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
mixeme