T3.2: Add app event types and observer dispatch

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>

src/app/service.go

@@ -27,6 +27,12 @@ type Service struct {
 	store    *storage.Store
 	jobs     []domain.Job
 	runtimes map[int]*domain.JobRuntime
+
+	// observers and their guard live in events.go. dispatchMu is separate from mu
+	// so that emitting an event never requires (or is held under) the state lock:
+	// the Service must release mu before dispatching, per the locking contract.
+	dispatchMu sync.Mutex
+	observers  []Observer
 }
 
 // NewService wires the Service to a loaded store and its jobs. It builds the