Speed up NYT::Format

Profile-driven optimizations of the `Format` hot path, benchmarked against a representative master debug log (structured `"Key: %v"` messages dominated by GUIDs, strings, integers, bools and durations). Median improvements of ~15-20% across the workload, measured on a dedicated host.

Changes:
- `string_builder`: use `resize_uninitialized` in `DoReserve` to avoid zero-filling the buffer on every `Format` call.
- `format`: replace the per-argument `memchr` (`spec.Contains('n')`) with an inline scan, force-inline `RunFormatterAt`, and add a `FormatString` fast path for the common plain `%v` / empty spec.
- `guid`: rewrite `WriteGuidToBuffer` using a `clz`-derived digit count and a back-to-front fill instead of the per-magnitude branch cascade (cut from ~26% to ~12% of a GUID-heavy line). Validated against an `%x` reference over 2M random GUIDs plus edge cases.

Also adds `library/cpp/yt/string/benchmark` to track `Format` performance.

### Benchmarks

Median ns/op (lower is better), pinned core on a dedicated Xeon E5-2650 v2, 9x1s repetitions. See `library/cpp/yt/string/benchmark`.

| Benchmark | What it formats | Before | After | Speedup |
| --- | --- | ---: | ---: | ---: |
| `ManyMixedArgs` | ~18 args: GUIDs, strings, duration, ints | 1030 | 833 | -19% |
| `StringAndTwoGuids` | literal prefix + two GUIDs | 233 | 185 | -21% |
| `IntAndGuid` | one int + one GUID | 205 | 179 | -13% |
| `ManyInts` | six integers | 389 | 340 | -13% |
| `Guid` | a single GUID | 156 | 131 | -16% |
| `String` | a single string | 139 | 104 | -25% |
| `Int` | a single integer | 142 | 120 | -15% |
| `NoArgs` | a literal with no arguments | 88.8 | 85.7 | -3% |
commit_hash:ce9957a06c3ff28b2889aa65fbbddf4ca444f9fe

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