feat: restore optional hotpath profiling

[fanyang89]

Summary

• restore optional hotpath-rs 0.19 support behind the hotpath feature, keeping default builds dependency-free via local no-op macros
• instrument hot-path locks, channels, and functions with hotpath wrap/mutex/rw_lock/channel/measure coverage
• initialize the profiler from easytier-core only when hotpath is enabled, with hotpath-alloc rejected alongside jemalloc or mimalloc

Notes

• parking_lot::Mutex remains unwrapped because hotpath 0.19 only exposes parking_lot RwLock wrappers
• peer_ospf_route parking_lot RwLock remains unwrapped because it uses upgradable_read guards that hotpath does not expose; route lookup still has function-level measure coverage
• quic RwPool Debug is manual because hotpath wrapped RwLock does not implement Debug

Benchmark

Short local Criterion run, ring tunnel, 1400B packets, inflight 64, 4 workers:

case	off	hotpath on	delta
ring	5.01 us / 266 MiB/s	5.35 us / 250 MiB/s	+6.3% time
ring-saturate	4.99 us / 268 MiB/s	5.43 us / 246 MiB/s	+9.3% time

This confirms default/off stays baseline while enabled profiling carries the expected measurement overhead.

Validation

• cargo check -p easytier --all-targets
• cargo check -p easytier --features hotpath --all-targets
• cargo build -p easytier --features hotpath,hotpath-cpu --bin easytier-core
• cargo clippy -p easytier
• cargo clippy -p easytier --features hotpath
• cargo check -p easytier --features hotpath-alloc,jemalloc confirms compile_error
• cargo check -p easytier --features hotpath,hotpath-alloc --lib
• cargo test -p easytier --lib -- --skip tests::
• cargo test -p easytier --lib secure_datagram

Full integration tests requiring brctl/network namespaces were not runnable in this environment.