Add library/cpp/yt/rseq: NYT::GetCurrentCpuId() via Linux rseq

Self-contained current-CPU-id reader backed by Linux **rseq** (restartable
sequences), with **no third-party dependency** (no librseq):

* The rseq ABI is hand-defined; the calling thread is registered lazily via the
  rseq syscall.
* Fast path is a single inlined, **branch-free** thread-local read. The offset
  always points at a readable `cpu_id` -- the glibc-owned area when glibc registers
  rseq (>= 2.35, via the weak `__rseq_offset`/`__rseq_size`), otherwise our own
  area -- so an unregistered thread reads `-1` and routes to the slow path.
* Falls back to `sched_getcpu()` (Linux) or `0` (darwin/windows). Works on glibc
  **and musl** alike (librseq does not build on musl).

Fiber-TLS contract: the inlined read must be reached only via a non-inlinable,
fiber-switch-free frame (a virtual call or `YT_PREVENT_TLS_CACHING`).

#### Benchmark -- cost of one cpu-id read

| source | time / call |
|---|---|
| `GetCurrentCpuId()` (rseq) | **0.34 ns** |
| `sched_getcpu()` (vDSO) | 3.5 ns |
| `rdtscp` (what `TTscp::Get()` does) | 23 ns |

This is an alternative to the librseq-based review/13886037 -- same speed, but no
contrib dependency and it also covers musl. The unit test pins to each allowed CPU
and asserts the reported id matches.

🤖 Generated with [Claude Code](https://claude.com/claude-code)
commit_hash:09d282c2f48755836b1cd68cedbffc3c6a662eed

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