feat: Add incremental task caching (#12531)
> [!NOTE]
> This is an experiment and we're not documenting it yet. We want to
learn about what this might look like. Use at your own risk.
## Summary
Adds incremental cache partitions that persist tool-managed incremental
state (e.g. `.tsbuildinfo`, Rust `target/incremental/`) across runs via
remote cache. On cache misses, turbo restores prior incremental state
before the tool runs, enabling faster re-execution. Gated behind
`futureFlags.incrementalTasks`.
- New `incremental` field on task definitions in `turbo.json`
- `IncrementalTaskCache` in `turborepo-run-cache` handles fetch/upload
lifecycle
- Cache keys use SHA-256 with length-prefixed encoding (prevents
separator collisions)
- Sequential fetch (deterministic last-write-wins overlap), concurrent
upload
- 30s fetch / 60s upload timeouts prevent slow cache from blocking tasks
- Graceful degradation: all failures are warnings, never task failures
### Notable design decisions
- **Upload on every success**: This might be overdoing it, but we are
going to try and find out to see what it looks like.
### How to test
1. Enable the feature flag: `"futureFlags": { "incrementalTasks": true
}` in root `turbo.json`
2. Add `incremental` to a task:
```json
"check-types": {
"incremental": [{ "outputs": ["tsconfig.tsbuildinfo"] }]
}
```
3. Run the task twice — second run on a cache miss should log
"incremental state restored"
4. Test with `--force` (skips fetch, still uploads), `--no-cache`
(disables entirely), `--remote-only` (skips local file check)
