wip(probe): throwaway iterate-iterate-2AZ-vsync probe instrumentation

Uncommitted experimental probe code preserved for handoff. Per running
memory these probes are inert/throwaway diagnostics, not production fixes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/xenia-cpu/src/scheduler.rs

@@ -1196,6 +1196,26 @@ impl Scheduler {
         }
     }
 
+    /// Maximum guest timebase across every thread in every slot's runqueue
+    /// (2.AZ). This is the global guest-clock proxy: it advances both when
+    /// any thread executes (per-instruction `timebase += 1`) and when the
+    /// idle path jumps the timebase forward to a pending deadline
+    /// (`advance_all_timebases_to`). Unlike `ctx(hw_id).timebase` — which
+    /// reads only the *currently scheduled* thread on one slot and therefore
+    /// stalls whenever that slot's thread is Blocked — the max is monotone
+    /// across the whole machine, so a v-sync ticker keyed to it keeps
+    /// advancing even when the slot-0 thread is wedged. Deterministic:
+    /// derived purely from guest-cycle state, never host wall-clock.
+    /// Returns 0 when no threads exist.
+    pub fn max_timebase(&self) -> u64 {
+        self.slots
+            .iter()
+            .flat_map(|slot| slot.runqueue.iter())
+            .map(|t| t.ctx.timebase)
+            .max()
+            .unwrap_or(0)
+    }
+
     /// Fast-forward the timebase to the earliest pending timed wait and
     /// wake that sleeper. Used when a round had no Ready threads and no
     /// timer fires closer than the earliest wait. Returns the woken