feat(infra/clock): injectable Clock with Real and Fake implementations

internal/infra/clock/clock.go

@@ -0,0 +1,18 @@
+// Package clock provides an injectable time source. Production code uses Real();
+// tests use NewFake(t) to make time-based logic deterministic.
+package clock
+
+import "time"
+
+// Clock is the abstraction over the system clock. Implementations must be
+// safe for concurrent use.
+type Clock interface {
+	Now() time.Time
+}
+
+type realClock struct{}
+
+// Real returns a Clock backed by time.Now.
+func Real() Clock { return realClock{} }
+
+func (realClock) Now() time.Time { return time.Now() }