perf: Reuse archive anchor during cache creation (#14010)
Fixes TURBO-5987
Reuse one verified cache-archive root descriptor and canonical root path
across all entries in a `CacheWriter`. Child components continue to use
the existing `openat`/`O_NOFOLLOW` traversal and internal-symlink
validation.
## Performance
Measured an end-to-end local cache miss for a no-op task with 10,000
existing tiny outputs under a depth-5 directory. Each sample removed
`.turbo`, ran the release-profile Turbo binary with `--daemon=false`,
and wrote a fresh local cache archive. The generated fixture and
benchmark artifacts remained outside the repository.
`hyperfine 1.19.0`, 3 warmups and 5 measured samples per binary in each
of four balanced AB/BA blocks (20 samples per binary), Apple M4 Max, 36
GiB RAM, macOS, `rustc 1.98.0-nightly`.
| Metric | Before | After | Improvement |
| --- | ---: | ---: | ---: |
| Pooled median save latency | 1,499.00 ms | 1,393.81 ms | 7.02% |
| Sample standard deviation | 20.84 ms | 18.29 ms | 12.2% lower |
| Block 1 median | 1,507.20 ms | 1,410.39 ms | 6.42% |
| Block 2 median | 1,494.84 ms | 1,388.85 ms | 7.09% |
| Block 3 median | 1,481.27 ms | 1,393.04 ms | 5.96% |
| Block 4 median | 1,498.18 ms | 1,394.59 ms | 6.91% |
Command:
```sh
hyperfine --warmup 3 --runs 5 --prepare 'rm -rf .turbo' \
'/tmp/turbo-baseline run build --daemon=false --no-update-notifier >/dev/null' \
'/tmp/turbo-candidate run build --daemon=false --no-update-notifier >/dev/null'
```
The candidate improved in every command order and every block. This
workload isolates the many-file archive-save case; it does not claim the
same percentage for cache restores or large individual files.
