next: scope turbo.json inputs for build task
The build task declared no explicit inputs, so turbo defaulted to hashing
every file in the package. Two practical consequences:
1. Editing docs, tests, or stories in the next package invalidated the
build cache.
2. packages/next/src/build/swc/generated-native.d.ts, which is
re-generated by scripts/build-native.ts whenever napi bindings
change, silently got rewritten mid-build and killed cache for the
next pnpm build run.
Narrow the inputs to src and the build-config files, and exclude tests,
stories, snapshots, fixtures, docs, and the generated d.ts. Warm reruns
now hit turbo's cache (<1s) instead of rebuilding from scratch.
pnpm --filter=next build (bypasses turbo) is unaffected.
