[tldraw] Warn when duplicate bindings are created

## Problem SDK users calling `editor.createBindings()` directly can accidentally create multiple bindings with the same `(fromId, toId, type)` tuple. This was silently tolerated in older versions (up to 4.3) but now causes broken behavior — particularly for arrow bindings where duplicate terminals lead to rendering issues. Diagnosing this is difficult because tldraw gives no feedback when duplicates are created. The `arrowBindingsCache` uses `.find()` which silently picks the first match per terminal, masking the problem. No single PR was identified that explicitly changed the duplicate binding behavior — it's likely a side effect of one of several changes between v4.3 and v4.4 (candidates: #7733 perf optimization in ArrowBindingUtil, #7821 passing shapes to canBind). ## Proposed solution Add a `console.warn` in `Editor.createBindings()` that checks for existing bindings in the store with the same `(fromId, toId, type)` before creating new ones. - Detection point: `Editor.createB