YT-28458: Add rseq-backed hot sensors

Rename `percpu.{cpp,h}` to `per_cpu.{cpp,h}` and update all includers.

Add an rseq-backed implementation of the hot per-CPU counter, time counter and
gauge (`rseq_sensor_impl.{h,cpp}`): each update commits to the calling CPU's shard
lock-free via an rseq critical section (`library/cpp/yt/rseq`
`AddPerCpu`/`StorePerCpu`) — no atomic and no lock on the fast path. The shard
array is sized to `NRseq::GetCpuCount()` and folded into the sensor allocation
(`NewWithExtraSpace`); reads aggregate with `LoadPerCpu`. The gauge keeps
last-writer-wins semantics. Linux-only.

The existing `TPerCpu{Counter,TimeCounter,Gauge}` stay (`per_cpu_sensor_impl.{h,cpp}`)
as the atomic sharded fallback. The two are interchangeable and chosen per sensor
at construction in `TSolomonRegistry`, so the hot Increment/Update path carries no
dispatch:

* The rseq fast path is **off by default**; opt in via
  `singletons/solomon_registry/enable_rseq`. Even when on, a hot sensor uses it
  only in a process where the kernel rseq area sits at a fixed thread-pointer
  offset (tcmalloc/glibc-owned), per the rseq library's runtime safety probe
  (`NRseq::IsPerCpuFastPathSafe`). Everything else — notably a `dlopen`'d YQL UDF
  whose `__rseq_abi` lands in dynamically allocated TLS — uses the atomic sharded
  sensors.
* `TSolomonRegistry` is a reconfigurable singleton (`solomon_registry`) with an
  `enable_rseq` knob (default false), settable in static config and updatable via
  dynamic config.

Off Linux the rseq sensors do not exist and the registry uses the atomic sharded
sensors for hot requests. The per-CPU summary is unchanged (TTscp + spinlock).
Unit tests cover the atomic sensors, the rseq sensors (Linux-only), and the simple
sensors. The controller-agent memory-watchdog integration tests are made resilient
to the (core-count-dependent) per-sensor footprint.

Benchmark (hot per-CPU path, 64-core host), atomic sharded vs rseq:

| Benchmark | atomic | rseq | speedup |
| --- | --- | --- | --- |
| BM_PerCpuCounter, threads:1  | 30.7 ns | 3.6 ns  | ~8.5x |
| BM_PerCpuCounter, threads:16 | 30.9 ns | 4.4 ns  | ~7x   |
| BM_PerCpuGauge, threads:1    | 32.8 ns | 12.4 ns | ~2.6x |
| BM_PerCpuGauge, threads:16   | 32.7 ns | 12.5 ns | ~2.5x |
commit_hash:8c633d31f03b2cc862ed2217ae08342bf42adc52

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