Remove webpack stats generation from Turbopack (#91515)
### What?
Remove the `TURBOPACK_STATS` feature that generated webpack-compatible
`webpack-stats.json` files from Turbopack's output.
**Deleted:**
- `crates/next-api/src/webpack_stats.rs` — the entire 305-line Rust
implementation that built webpack-format stats (assets, chunks, modules,
entrypoints) from Turbopack's module graph
**Rust changes:**
- `crates/next-api/src/lib.rs` — removed `mod webpack_stats;`
- `crates/next-api/src/project.rs` — removed
`should_create_webpack_stats()` which read the `TURBOPACK_STATS` env var
- `crates/next-api/src/pages.rs` — removed `generate_webpack_stats`
import and stats output emission
- `crates/next-api/src/app.rs` — same
**TypeScript changes:**
- `packages/next/src/shared/lib/constants.ts` — removed `WEBPACK_STATS =
'webpack-stats.json'` constant
- `packages/next/src/shared/lib/turbopack/manifest-loader.ts` — removed
`loadWebpackStats()`, `writeWebpackStats()`, `mergeWebpackStats()`, the
`webpackStats` field, webpack type imports, and the `TURBOPACK_STATS`
env guard in `writeManifests()`
- `packages/next/src/server/dev/turbopack-utils.ts` — removed
`shouldCreateWebpackStats` flag and two `loadWebpackStats()` call sites
- `packages/next/src/build/handle-entrypoints.ts` — same
### Why?
This feature is no longer needed and adds dead complexity. It was opt-in
via `TURBOPACK_STATS=1` but nothing in the current codebase or tooling
depends on it.
### How?
Pure deletion — no behavior change for any existing user, as the feature
was opt-in via an environment variable that is no longer checked.
**Intentionally left untouched:**
- Webpack's own `stats.json` generation (non-Turbopack path)
- The Turbopack bundle analyzer / `analyze` feature
- `route-bundle-stats.json` in `.next/diagnostics/`
Co-authored-by: Tobias Koppers <sokra@users.noreply.github.com>
Co-authored-by: Claude <noreply@anthropic.com>
