perf(core): delegate observation instead of attaching listeners per element

[vamgan]

Summary

Replace per-annotated-element interaction listeners with delegated observation at the configured root, while preserving target strategy, priority, touch, scope, and per-element event semantics.

Evidence

packages/core/src/observer.ts currently scans all [data-askable] elements and attaches each active DOM event to each element. With default click/hover/focus behavior, 10,000 annotated elements create approximately 30,000 listeners plus a subtree MutationObserver.

Existing benchmarks pass, but they measure elapsed initialization and event latency—not listener count, heap retention, teardown pressure, or repeated dynamic churn.

Proposed direction

• Delegate bubbling click and focusin at the root.
• Replace mouseenter with delegated mouseover/pointerover, filtering relatedTarget to preserve enter semantics.
• Resolve candidates through the composed event path for Shadow DOM support.
• Keep element discovery only where viewport tracking needs IntersectionObserver registration.
• Preserve the current public API.

Acceptance criteria

• Listener count is O(active DOM events), independent of annotation count
• Deepest/shallowest/exact targeting remains compatible
• Priorities and data-askable-events remain compatible
• Touch hover mapping remains compatible
• Shadow-root behavior has browser tests
• Real-browser benchmarks cover 1k/10k/50k annotations, heap, teardown, and dynamic churn
• Current performance budgets remain green