chore(core): drop fixtures from published npm package

[ignsm]

Fixtures (packages/core/fixtures/*.json) were shipped with the npm tarball but never reachable from consumer code: exports only declared the root entry, so import x from '@formhaus/core/fixtures/basic-form.json' fails under strict ESM (Node 16+, Vite production, Next.js prod build) with ERR_PACKAGE_PATH_NOT_EXPORTED. Two options were on the table:

1. Add ./fixtures/*.json to exports, keep shipping them
2. Drop them from the tarball, treat them as repo-internal demo data

This PR takes (2). Reasons:

• They're demo data for the docs and playground, not API surface. Consumers write their own definitions; nobody real builds an app on @formhaus/core/fixtures/basic-form.json.
• Coupling consumer code to internal fixture file names creates a future versioning risk: rename dispute-form.json → bumps anyone using it.
• The Quick Start guide that pointed at @formhaus/core/fixtures/basic-form.json was broken in production builds anyway. Now it shows an inline definition (which is what the README and every other docs page already do).

Changes

• packages/core/package.json: removed "fixtures" from files. Tarball now contains dist/, package.json, README.md, LICENSE only.
• docs/guide/index.md: Quick Start now uses an inline definition matching root README's style.
• .changeset/drop-fixtures-from-npm.md: patch on @formhaus/core.

Fixtures still live in the repo at packages/core/fixtures/ for the playground and reference. They're just not part of the published package.

Verifying

cd packages/core && pnpm pack
tar -tzf formhaus-core-0.3.1.tgz
# package/dist/index.js
# package/package.json
# package/README.md
# package/dist/index.d.ts
# package/LICENSE

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Project	Deployment	Actions	Updated (UTC)
formhaus	Ready	Preview, Comment	May 10, 2026 2:30pm