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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