Initial iris repository structure

WASM reactive UI framework for Go:
- reactive/ - Signal[T], Effect, Runtime
- ui/ - Button, Text, Input, View, Canvas, SVG components
- navigation/ - Router, guards, history management
- auth/ - OIDC client for WASM applications
- host/ - Static file server

Extracted from arcadia as open-source component.

Co-Authored-By: Claude <noreply@anthropic.com>

reactive/runtime.go

@@ -0,0 +1,39 @@
+package reactive
+
+import "sync"
+
+type Runtime struct {
+	signalValues      []any
+	runningEffect     *EffectId
+	signalSubscribers map[SignalId][]EffectId
+	effects           []func()
+}
+
+var lock = &sync.Mutex{}
+var runtimeInstance *Runtime
+
+func GetRuntime() *Runtime {
+	if runtimeInstance == nil {
+		lock.Lock()
+		defer lock.Unlock()
+		if runtimeInstance == nil {
+			runtimeInstance = newRuntime()
+		}
+	}
+	return runtimeInstance
+}
+
+func newRuntime() *Runtime {
+	return &Runtime{
+		signalValues:      []any{},
+		signalSubscribers: map[SignalId][]EffectId{},
+		effects:           []func(){},
+	}
+}
+
+func runEffect(rt *Runtime, id EffectId) {
+	previous := rt.runningEffect
+	rt.runningEffect = &id
+	rt.effects[id]()
+	rt.runningEffect = previous
+}