fix(next): accept interface-typed next config in withPrewire

[clroot]

Problem

withPrewire<C extends Record<string, unknown>> rejects NextConfig:

error TS2345: Argument of type 'NextConfig' is not assignable to parameter of
type 'Record<string, unknown> | NextConfigFn<Record<string, unknown>>'.
  Type 'NextConfig' is not assignable to type 'Record<string, unknown>'.
    Index signature for type 'string' is missing in type 'NextConfig'.

NextConfig is an interface, and interfaces get no implicit index signature.
The README's own snippet (withPrewire({ reactStrictMode: true })) passes only
because an object literal is inferred as an anonymous type — the moment an app
does the normal thing:

const nextConfig: NextConfig = { ... }
export default withPrewire(nextConfig)

it fails to typecheck. Hit while adopting prewire in a real Next 16 monorepo.

Fix

C extends object. The constraint only exists to exclude primitives; the
structural Record requirement buys nothing and excludes every interface.

The regression test deliberately declares an interface-typed config — an object
literal passes under either constraint, so it would not have caught this.

Also

README gets a section on resolving #prewire when shared packages are consumed
as source (transpilePackages and friends), which the current docs do not
cover. Two findings from the migration:

• #… subpath imports are package-local by spec, so a #prewire inside
  packages/kit/src cannot resolve to the importing app's root via
  package.json imports. The app's tsconfig paths is sufficient and
  Turbopack honours it for transpiled workspace sources — no bundler alias
  needed (verified on Next 16.1.4, next build, 3 apps).

• tsconfigPaths: true emits prewire.paths.gen.json for the app tsconfig to
  extends, but TypeScript replaces compilerOptions.paths across
  extends instead of merging. Any app with its own @/* alias silently loses
  the generated mapping. Documented the tsconfigPaths: false + inline entry
  route.

  Worth considering separately: codegen already patches package.json
imports in place when conditions is set; doing the same for a
  user-named tsconfig would close the gap, but tsconfig is frequently JSONC
  and a comment-preserving rewrite is not free. Docs felt like the honest
  fix for now.

Verification

bun run lint, bun run build, bun run test, prettier --check . — all
green. @prewire/next tests 4 → 5.