feat(plugin-nextjs): add opt-in Turbopack support

[dmiller1-godaddy]

Summary

Adds opt-in Turbopack handling to @gasket/plugin-nextjs while preserving the existing Webpack behavior by default.

Next.js 16 defaults to Turbopack, but the plugin currently relies on a Webpack callback and NormalModuleReplacementPlugin rules for server-only Gasket files. Consumers can now set TURBOPACK=1 alongside the Next CLI --turbopack flag.

Changes

• Adds a nextConfig hook gated by TURBOPACK=1.
• Removes the plugin's Webpack callback only in Turbopack mode.
• Adds @gasket/core and loaded @gasket/plugin-* / @*/gasket-plugin-* packages to serverExternalPackages.
• Preserves and deduplicates consumer-provided server externals.
• Documents the opt-in behavior and app-owned alias/fallback responsibilities.
• Adds a minor changeset and regression coverage.

Validation

• pnpm --filter @gasket/plugin-nextjs test: 167 tests passed.
• Package lint passed with six pre-existing lib/index.d.ts warnings.
• Package typecheck passed.
• pnpm --filter @gasket/plugin-nextjs build passed; 17 CJS files generated.
• Direct config probes confirmed:
  • default mode returns the original config with Webpack intact;
  • TURBOPACK=1 removes Webpack and adds deduplicated Gasket server externals.

Downstream validation

This behavior was exercised in a Next.js 16.2.6 App Router application using Turbopack, including server routes, Presentation Central, and a full browser smoke test. The downstream integration remains opt-in; app-specific Webpack aliases/fallbacks still need equivalent turbopack.resolveAlias configuration.

[changeset-bot]

🦋 Changeset detected

Latest commit: 866092b

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@gasket/plugin-nextjs	Minor

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