fix(build): restore exports field in dist/package.json

[Olovyannikov]

Problem

build.mjs generates dist/package.json from a whitelist of fields, but uses a typo export: Existing.export (singular, non-existent) instead of exports: Existing.exports (introduced in cef40e6). As a result the published 10.1.0 package has no exports field.

Impact

• The @effector/reflect/scope subpath no longer resolves.
• Bundlers fall back to main (CJS). The CJS entry requires its own copy of effector-react, causing a dual-package hazard: <Provider> (ESM) and useUnit inside variant/reflect (CJS) reference different effector-react instances, so the forked scope is lost — variant reads the store default instead of the scoped value.

Reproduced in a React 19 app: variant({ if: $store, then, else }) under <Provider value={fork(...)}> rendered the then branch even though the scoped value was false. Pinning back to 10.0.2 (which still shipped exports) fixed it.

Fix

• build.mjs: export → exports.
• validate_dist.mjs: fail when the generated dist/package.json is missing the exports field (. / ./scope). The existing attw/publint calls swallow their errors, so the regression went unnoticed.

Verify

pnpm build now emits dist/package.json with the full exports map (both . and ./scope conditions).

Closes #104

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[kireevmp]

Hey there! Thanks for taking the time to open this.

I think this is good to go either way as a fix, but would appreciate if you can take a look at a comment I left below.

[kireevmp]

I believe publint can actually do the work of checking exports for us. This generates a "suggestion" but is plenty informative to us.

pkg.module is used to output ESM, but pkg.exports is not defined. As NodeJS doesn't read pkg.module, the ESM output may be skipped. Consider adding pkg.exports to export the ESM output. pkg.module can usually be removed alongside too.

Would you be open to swapping the zx call for publint's Node API? Something similar to

const result = await publint({ pkgDir, strict: true }); // import from 'publint'
if (result.messages.length > 0) throw new Error(/* message */)

This would removing manual "exports" check in favor of a supported tool we already use, but didn't gate on.