Local downloading of optimization statistics for the build

> Implementing at #1186

Enables local download and reuse of optimization stats, supporting filter creation based on a pinned stats snapshot.

This feature is managed entirely through build flags in FiltersRegistry.

### The workflow

- `yarn download-stats` (`--download-stats`) downloads per-filter `stats.json` files. Use `--include` or `--skip` to scope which filters are downloaded.
- `yarn build:local`: If downloaded optimization stats exist, stats are read from there; if any required filter is missing its `stats.json`, the build fails with a message pointing to `yarn download-stats`. If `stats` doesn't exist, stats are fetched live from the remote server instead.