debug: instrument StallWatchdog timing for a Windows CI trace

TEMPORARY. Prints the actual clock/backoff numbers on every poll so the
Windows job's log can be compared against Linux's, instead of guessing.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

core/include/stplugin/session_types.h

@@ -202,6 +202,13 @@ public:
     /// onFrameDelivered and by setExpectingFrames.
     int attemptsThisStall() const { return attempts_; }
 
+    // TEMPORARY DEBUG ACCESSORS -- remove before merge.
+    long long dbgBackoffMs() const { return (long long)backoff_.count(); }
+    long long dbgTimeoutMs() const { return (long long)timeout_.count(); }
+    long long dbgMaxBackoffMs() const { return (long long)max_backoff_.count(); }
+    std::chrono::steady_clock::time_point dbgNextAllowed() const { return next_attempt_allowed_; }
+    std::chrono::steady_clock::time_point dbgBaseline() const { return baseline_; }
+
 private:
     std::chrono::milliseconds timeout_;
     std::chrono::milliseconds max_backoff_;