Turbopack: aggregate server HMR into one subscription
Replace per-chunk Server HMR fan-out with a single firehose subscription that diffs every HMR-eligible chunk under the target root in one tick. This significantly cuts the number of tokio task churn on projects with many server chunks and centralizes the diff/clear logic. It leads to a multi-second saving in both cold and warm builds in a large app.
A following PR will bring this to client chunks.
Rust:
- New `aggregate_hmr` module: `AggregateHmrVersion` keyed by chunk path, `merged_partial_update` builder, and `is_hmr_eligible_chunk` (excludes `.map` files, which would force every diff to `Total`).
- `Project::all_hmr_version_state` / `all_hmr_update` aggregate over the whole `hmr_root_path`. The seed transition emits an empty `Partial` so the JS consumer doesn't treat it as a restart and wipe handlers the triggering request just populated. Any chunk requiring `Total`/`Missing`
escalates the batch.
- `VersionedContentMap::hmr_chunks_in_path` lists eligible chunks with their `VersionedContent`.
NAPI:
- `projectAllHmrEvents(target)` returns a single subscription.
JS:
- `setupServerHmr` subscribes once via `allHmrEvents` instead of fanning out over `hmrChunkNamesSubscribe`.
- `clear()` evicts every chunk under `server/chunks/` from `require.cache` directly rather than tracking subscriptions. This is a bit fragile as it relies on the path prefix and scanning require.cache.
