Formisch and TanStack Form support

Summary

Track first-class shadcn-svelte form guides and registry examples for Formisch + Valibot and TanStack Form + Zod.

Scope

• Add a forms landing page and preserve the existing Formsnap guide
• Add Formisch and TanStack Form guides
• Port input, textarea, select, checkbox, radio group, switch, array, demo, and complex examples for both libraries
• Review both implementations and documentation against their respective shadcn/ui sources
• Migrate TanStack Svelte Form to 2.0.0-alpha.2
• Validate generated registry output across every style
• Run Svelte checking, linting, workspace tests, production builds, and browser interaction tests

Implementation is in draft PR #2843; this issue remains open until it is merged.

The alpha's array metadata subscription and indexed-field removal edge cases are handled in the examples and explained in the TanStack guide. Both prereleases have declaration-only NodeNext compatibility patches. See the PR description for the validation matrix and alpha-specific details.