perf: prefetch the bundle when the host page already loaded embed.js

[sidgaikwad]

Fixes #44.

1. The prefetch was skipped on host-injected pages (the real fix)

The load listener prefetches the versioned bundle so the first createEditor doesn't pay a second round trip — but the early return bypassed it:

if (window.ImageEditor) {
  return Promise.resolve();   // no prefetch
}

So on exactly the pages the tag-reuse logic exists to support — where the host injected embed.js itself — the prefetch never happened and the first mount was a full network round trip slower than the injected path.

The embed loader caches its own promise, so the duplicate call is a no-op when the host already triggered it. The rejection is swallowed identically to the existing listener, since the same failure resurfaces through createEditor.

Red without the fix:

× prefetches the bundle when the host page already installed the global

2. The reused-tag timeout

REUSED_TAG_TIMEOUT_MS is now exported and loadScript takes it as an optional second argument, so the bound is overridable and directly testable rather than requiring vi.advanceTimersByTime(30_000).

I deliberately stopped short of a component prop. loadScript isn't exported from the package entry point, so surfacing this to consumers means either exporting the module or adding a prop alongside scriptUrl — an API decision that's yours, not mine. This change makes either one a one-liner later. Say the word and I'll add the prop in this PR.

Verification

• 3 new tests; 49 total; coverage still 100% across the board
• lint, typecheck, build clean

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[lucasbesen]

I don’t think we should merge this as written.

On the normal mount path, createEditor() runs immediately after await loadScript() and starts the bundle request itself. Prefetching here moves that request slightly earlier; it doesn’t eliminate an additional network round trip.

It also changes version selection for host-installed embeds. With embed.js already evaluated and no bundle loaded, passing options={{ version: '2.6.0' }} loads 2.6.0 on the base branch but 2.7.0 on this branch. The new load() call selects “latest” first, and the cached promise prevents createEditor() from honoring the pin. The mocked tests don’t catch this.

Please remove the prefetch change, or demonstrate a meaningful performance improvement while preserving explicit version selection and adding regression coverage.

The timeout parameter is reasonable internal cleanup, but it remains inaccessible to package consumers, so it doesn’t address consumer configurability yet.