fix: make perf-self-profile compile on macOS

[yulnr]

Closes #14

Summary

Makes dial9-perf-self-profile compile on platforms other than Linux by re-organizing some of the implementation through splitting between platform specific and platform-agnostic code (and gating the linux-specific).

Note: I think that this approach could be a good idea long-term, it prevents lots of individual cfg gates everywhere, this might make it easier to maintain/extend without having to manually add cfgs frequently, and it should make adding support for other platforms later easier. That said it comes at the cost of some bigger diffs now, so I'm happy to switch to a smaller set of changes and more individual gating if that's preferred.

Breakdown

Moves Linux-specific code behind sys/linux/. On non-Linux, PerfSampler constructors return io::ErrorKind::Unsupported and everything else is a no-op.

Shared types (Sample, SymbolInfo, SymbolTableEntry, etc.) stay in their original files. The Linux impl. (PerfSampler, ringbuffer, resolve_symbol, offline symbolization) moves intosys/linux/.

In case it makes it easier to review, it now looks like this:

perf-self-profile/src/
  lib.rs
  sampler.rs                     # shared types: EventSource, SamplerConfig, Sample
  symbolize.rs                   # shared types: MapsEntry, SymbolInfo, CodeInfo
  offline_symbolize.rs           # SymbolTableEntry + cross-platform entry point
  sys.rs                         # cfg dispatch
  sys/
    linux.rs                     # mod declarations, USER_ADDR_LIMIT
    linux/
      sampler.rs                 # PerfSampler, perf_event_open, ring buffer consumer
      ring_buffer.rs            # moved from src/
      symbolize.rs
      offline_symbolize.rs
    unsupported.rs               # stubs returning Unsupported

While at it, fixed a few things that caused warnings on macOS: the blocking_pool_tracking example was missing required-features, some items in events.rs/event_writer.rs needed cpu-profiling feature gating, and a couple test files were pulling in mod common unconditionally but they only use it behind feature gates.

[netlify]

✅ Deploy Preview for dial9-tokio-telemetry canceled.

Name	Link
🔨 Latest commit	7ab78eb
🔍 Latest deploy log	https://app.netlify.com/projects/dial9-tokio-telemetry/deploys/69d922ae0581950008afbb92

[rcoh]

LGTM. Do you mind adding something to the github actions matrix that verifies these changes to make sure we don't regress?