fix(dsn): limit concurrent stat() calls in project root detection (CLI-19A) (#768)

## Summary

- `processDirectoryLevel` fires up to 68 parallel `stat()` calls per
directory level through `anyExists()` (VCS: 7, CI: 12, language: 30,
build: 19) with no concurrency limit
- On macOS, this exhausts kqueue file descriptor limits for deeply
nested projects, surfacing as `EINVAL` (CLI-19A)
- Add a module-scoped `pLimit(32)` instance (`statLimit`) shared across
every `anyExists()` call

## Approach

The shared limiter bounds **total** FD pressure across all marker
groups, not per-group pressure — so 4 parallel `anyExists()` groups
still share a single 32-slot budget. Matches the pattern in
`response-cache.ts` (`const cacheIO = pLimit(CACHE_IO_CONCURRENCY)`).

32 keeps VCS (7) and CI (12) marker checks fully parallel while
throttling only the larger `LANGUAGE_MARKERS` (30) and
`BUILD_SYSTEM_MARKERS` (19) groups, staying well under macOS's ~256
kqueue FD ceiling.

## Changes

- `src/lib/dsn/project-root.ts` — add `STAT_CONCURRENCY = 32` exported
constant + shared `statLimit`, wrap `pathExists` in `anyExists()` with
it
- `test/lib/dsn/project-root.test.ts` — add concurrency cap test (mocks
`stat` to track peak concurrent calls, asserts ≤ `STAT_CONCURRENCY`) and
early-exit preservation test

Author: BYK