perf: O(n²) membership in sort_const_tags topological sort

Context

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

Problem

• File:Line: packages/svelte/src/compiler/phases/3-transform/utils.js:92
• Pattern: linear membership checks during DFS topological sort
• Current complexity: O(n²) where n = number of {@const} tags in a fragment
• What n represents: Count of interdependent {@const} declarations in one fragment (can grow in complex templates)

function add(tag) {
  if (sorted.includes(tag.node)) {
    return;
  }
  // ...
  sorted.push(tag.node);
}

Why not in #18602

That PR fixed a related Map-misuse on legacy $: statements. This is the same pattern class on a different surface ({@const}).

Suggested direction

Track visited const tags with a Set (or Map) of nodes; O(1) membership. Preserve topological order and cycle detection.

Acceptance sketch

• Same public compile output for {@const} ordering samples (const-tag-ordering, const-tag-depends-on-const-tag)
• Membership checks O(1); total sort O(n + edges)