What does this PR do?:

Adds a native JFR deadlock event (datadog.DeadlockedThread) that captures real
JVMTI stacktraces for both the deadlocked thread and the lock owner. This provides
proper navigable stacktraces in JFR tools — a significant improvement over the
string-based stacks in the dd-trace-java fallback implementation.

Key components:

• New T_DEADLOCK JFR event type with fields: deadlockId, eventThread, stackTrace,
  lockName, lockOwnerThread, lockOwnerStackTrace
• JVMTI GetStackTrace() capture for both threads in the deadlock pair, stored in
  the profiler's CallTraceStorage as real stacktrace constant pool references
• DeadlockDetector Java class using ThreadMXBean.findDeadlockedThreads() with
  periodic polling on a daemon thread
• JavaProfiler.startDeadlockDetector() / stopDeadlockDetector() lifecycle API

Motivation:

PROF-11534 — The existing deadlock event in dd-trace-java uses JDK JFR with string
stacktraces. This PR adds the improved native implementation in java-profiler that
captures real stacktraces via JVMTI, which dd-trace-java can use when the profiler
is loaded.

Additional Notes:

• dd-trace-java keeps its existing DeadlockEventFactory as a fallback for
  environments where the profiler isn't loaded
• Works on JDK 8+ (all APIs are JDK 1.5+; JVMTI is always available)
• One event per deadlocked thread; events in the same cycle share a deadlockId
• Lock name truncated to 160 chars to fit within single-byte JFR event size prefix
• Supports both monitor and java.util.concurrent lock deadlocks (falls back to
  findMonitorDeadlockedThreads() when synchronizer usage is not supported)

How to test the change?:

./gradlew ddprof-test:testRelease --tests '*DeadlockDetectionTest*'

The test creates a real deadlock between two threads, starts the detector with a
100ms interval, and verifies that datadog.DeadlockedThread events appear in the
profiler's JFR output with non-empty stacktraces and matching deadlock IDs.

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-11534

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