fix: add mutex to Owner for goroutine-safe linked list operations

[gfanton]

Fix data race in Owner linked list when signals are written from a different goroutine than the one that created the reactive graph.

Root: AddChild, DisposeChildren, OnCleanup, OnError, and context map access had no synchronization. When Signal.Write from goroutine B triggers recompute of a node created on goroutine A, new children are added to A's owner tree concurrently — corrupting the doubly-linked list (infinite loop → freeze).

Fix: Add sync.Mutex to Owner with snapshot-then-release pattern — lock to snapshot/detach, unlock before executing callbacks or disposing children. No deadlocks: parent lock is always released before acquiring child locks.

See owner_race_test.go, 4 tests covering AddChild vs DisposeChildren, OnCleanup vs Cleanup, concurrent context access, and OnError vs recover. All fail with -race before the fix.