…very

Three targeted optimizations to the turbo run hot path:

1. Engine builder: Cache turbo.json chain per package and move the
   visited check before the expensive task_definition() call. The
   chain only depends on the package name, so multiple tasks in the
   same package reuse the cached result.

2. Task visitor: Defer env() computation to non-dry-run branches.
   The execution environment is unused during dry runs, avoiding
   per-task RwLock acquisition and env var map cloning.

3. find_untracked_files: Replace Mutex<Vec> with per-thread local
   buffers flushed via mpsc channel on drop, eliminating per-file
   mutex contention in the parallel walker.

Parallelize several sequential phases of turbo run's pre-execution
pipeline: dependency resolution, turbo.json loading, and task summary
construction. Also reduce per-call allocation overhead in the task
hash tracker and gix index classification.

…parallelize-hot-path

# Conflicts:
#	crates/turborepo-scm/src/repo_index.rs

…new refactor

…parallelize-hot-path

Adds profiling visibility to functions that were invisible in
--profile output: TLS initialization, rayon-spawned hash tasks,
Visitor constructor, Engine scheduler, and per-task cache phases.