What does this PR do?:
Intercepts libc send/recv calls via PLT hooking and records
datadog.NativeSocketEvent JFR events with byte-weighted inverse-transform
sampling (PID rate control, target ~5000 events/min).

Motivation:
Track blocking TCP socket I/O at the libc function level to surface socket
latency and throughput in Datadog profiler. Netty with Java NIO transport
(the primary use case) goes through libc send/recv, making PLT patching
the right interception point. Feature is explicitly opt-in via the
nativesocket profiler argument. Implements PROF-10637.

Additional Notes:

Key design decisions:

• PLT-hooks send/recv only (TCP blocking I/O); UDP sendto/recvfrom
  and Netty native epoll/io_uring are explicitly out of scope
• Hook bodies run on the calling Java thread, not in a signal handler —
  malloc and locking are safe inside hooks
• fd-to-remote-address cache is not cleared on JFR chunk boundaries:
  the same fd may remain valid across chunks and clearing would race with
  in-flight recordings
• _orig_send/_orig_recv are intentionally not nulled in
  unpatch_socket_functions to avoid a memory-ordering race with in-flight
  hook invocations on aarch64
• NativeSocketEvent struct lives in nativeSocketSampler.h (not
  event.h) because it is only used by NativeSocketSampler
• macOS: entire implementation compiled out as no-op stubs (#ifdef __linux__)
• Known limitation: dlopen-loaded libraries patched after start() are not
  intercepted (documented in libraryPatcher.h)

How to test the change?:

• GTest unit tests: NativeSocketSamplerHookTest in
  ddprof-lib/src/test/cpp/nativeSocketSampler_ut.cpp — verify that
  send_hook/recv_hook delegate to the installed _orig_send/_orig_recv
  function pointers
• 12 JUnit integration tests in
  ddprof-test/src/test/java/com/datadoghq/profiler/nativesocket/:
  • NativeSocketEnabledTest — events produced when feature is enabled
  • NativeSocketEventFieldsTest — all 8 required JFR fields present and valid
  • NativeSocketSendRecvSeparateTest — SEND and RECV events tracked independently
  • NativeSocketDisabledTest — no events when feature is not enabled
  • NativeSocketRateLimitTest — event count is substantially less than operation count (subsampling active), weight > 1 on sampled events
  • NativeSocketRemoteAddressTest — remoteAddress field is in ip:port format
  • NativeSocketMacOsNoOpTest — no events on macOS (no-op stub)
  • NativeSocketStackTraceTest — stack trace captured on events
  • NativeSocketBytesAccuracyTest — bytesTransferred field matches actual bytes
  • NativeSocketUdpExcludedTest — UDP sends do not produce events
  • NativeSocketEventThreadTest — eventThread field populated with calling thread
  • NativeSocketNettyNioTest — Netty 4.x with NioEventLoopGroup produces events

Spec: #486

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.
• JIRA: PROF-10637