What does this PR do?:

Before sending SIGVTALRM, the wall-clock timer checks whether the target thread is already in a skippable OS state and has already produced a sample during this blocking run. If so, the signal is suppressed. The first sample of every blocking run is always collected.

Motivation:

High-frequency wall-clock sampling on threads spending most of their time idle generates many redundant samples of identical stacks at high CPU cost. The precheck lets us emit one representative sample per blocking run and skip the rest, recovering signal throughput for threads that are actually running.

This is the first of two PRs splitting paul.fournillon/wallclock_precheck. This PR ships the suppression infrastructure. The follow-up PR adds datadog.TaskBlock event recording.

Additional Notes:

How to test the change?:

New unit tests:

• ddprof-lib/src/test/cpp/park_state_ut.cpp
• ddprof-lib/src/test/cpp/wallprecheck_args_ut.cpp

New integration tests:

• PrecheckTest: verifies suppression fires and reduces signal count.
• PrecheckEfficiencyTest: measures overhead reduction under sustained park.
• WallclockMitigationsCombinedTest: end-to-end combined mitigations scenario.

Run locally with:
./gradlew :ddprof-test:test --tests '*Precheck*' --tests '*WallclockMitigations*' --tests '*WallclockTimer*'

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: [JIRA-14354]

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