Correct ResizeObserver callback arity #387

Part of #382 — v1.0.0 alpha readiness.

Problem

src/observers/ResizeObserverTypes.res:54-L56 declares:

type resizeObserverCallback = array<resizeObserverEntry> => resizeObserver => unit

Native ResizeObserver invokes one callback with two arguments. The declared type instead makes callers return a second function. A compiled packed consumer emitted a nested function; invoking it with the native argument shape returned the inner function without executing its body. A real-frame Chromium probe confirmed the same behavior.

Acceptance criteria

• Change the callback contract to (array<resizeObserverEntry>, resizeObserver) => unit.
• Update affected fixtures and documentation examples.
• Add a real-browser regression that observes/resizes an element and asserts both callback work and access to the supplied observer.
• Add compile coverage that rejects the old nested callback shape where appropriate.
• Verify observation cleanup/disconnection so the test does not leak browser state.

Coordination

Use the regression infrastructure in #383. Source fixes can be developed in parallel, but this issue should close with executed regression coverage, not only a successful compile.