test: trip when the mirrored editor types move upstream

[sidgaikwad]

Partially addresses #42 — please read the caveat before merging.

What this does

src/types.ts hand-copies MountOptions, ImageEditorInstance, ImageEditorEmbed and ImageEditorSaveResult from the private @unlayer/image-editor package, with a comment saying it does so "until they move into @unlayer/types". Nothing enforced the "until".

Once that move happens, the local copies silently become a second, diverging source of truth. That's exactly how #25 (Features.imageEditor missing dock/corners) reached users.

test/typesDrift.test.ts scans the installed @unlayer/types declarations and fails the moment any mirrored name appears there:

AssertionError: @unlayer/types now exports JSONTemplate. Delete the mirrored
declaration(s) from src/types.ts and re-export from @unlayer/types instead,
then drop the name(s) from this test.

(That output is from deliberately adding a name that does exist upstream, to prove the tripwire fires — it passes as-is today, since @unlayer/types@1.477.0 exports none of the four.)

The KEEP IN SYNC comment now points at the test, so the enforcement is discoverable from the declarations.

What this deliberately does NOT do

It does not detect the private @unlayer/image-editor package changing underneath you — the more dangerous direction, where a renamed instance method typechecks fine and throws at runtime.

Of the three options in the issue:

1. Move the types into @unlayer/types — the durable fix, and the comment's own stated intent. Only Unlayer can publish that.
2. CI drift check against the private package — needs access this repository doesn't have.
3. Runtime shape assertion — I chose not to add one. It would mean loading the live CDN embed in CI, which makes the suite network-dependent and flaky for a check that only fires on a rename.

So this closes the cheap half and leaves the real fix as yours. Happy to do the @unlayer/types migration in a follow-up if you publish the types there.

Tests 47, coverage still 100%; lint, typecheck, build clean.

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