perf: O(k²) key dedup in spread_props ownKeys

Context

Found while reviewing runtime complexity in packages/svelte/src alongside https://github.com/sveltejs/svelte/pull/18602.

Problem

• File:Line: packages/svelte/src/internal/client/reactivity/props.js:238-252
• Pattern: quadratic key dedup via array includes while building ownKeys
• Current complexity: O(k²) where k = unique keys across all objects in a spread_props(...) list
• What n represents: Total enumerable string + symbol keys on spread prop objects (grows with large component spreads / design-system prop bags)

ownKeys(target) {
  const keys = [];
  for (let p of target.props) {
    // ...
    for (const key in p) {
      if (!keys.includes(key)) keys.push(key);
    }
    for (const key of Object.getOwnPropertySymbols(p)) {
      if (!keys.includes(key)) keys.push(key);
    }
  }
  return keys;
}

Why not in #18602

#18602 is a compile-time Map lookup fix. This is a separate runtime path and needs care around Proxy invariants and key order.

Suggested direction

Build uniqueness with a Set (or object-as-set for strings + Set for symbols), then return Array.from(set). Preserve first-seen order if that is currently load-bearing.

Acceptance sketch

• Same keys and order for Object.keys / Reflect.ownKeys on spread props
• No per-key linear scan of the growing list