fix(input): prevent validation layout shifts

[hruilabs]

Summary

• Add an opt-in reserveErrorLine prop so Input can keep one validation-message row without changing the default layout.
• Use the stable row in SignUpForm and the Input preview, with compact field spacing.
• Document the public behavior, refresh affected registry dates, and cover the stable message row with a focused test.

Verification

• bun test tests/input.test.tsx: 4 passed
• bun run check: typecheck and lint passed; 79 registry components validated
• Cypress against the real local Sign Up Form page: four field errors appeared without moving any input relative to the form; input-to-next-label gap was 27px and field-container gap was 4px.

[starc007]

Thank you for the contribution — the implementation is clean, focused, and well validated. Merging this now.