fix(tui): soften theme scale extremes

[jlongster]

Summary

• move default text and background roles one step inward on V2 theme scales
• anchor migrated V1 neutral and chromatic colors at steps 200/800
• extrapolate outer scale steps safely, including transparent and achromatic colors
• align the form prompt border with the mode-specific interactive anchor

Testing

• bun typecheck in packages/tui
• bun test test/theme/v2/v1-migrate.test.ts in packages/tui
• bun test test/cli/tui/form.test.tsx in packages/tui