[iterate-3R] Fix --ui boot throttle: demote idle-advance scheduler log to DEBUG

The publisher-splash draws first appear ~30M instructions and ramp through
80M-150M. Headless `check` reaches that window and renders the textured logo
(DRAW_INDX=568, gpu.texture.decode{K8888}=279 at -n 150M). The interactive
`exec --ui` path appeared to "self-terminate at ~8.8s / ~33M" before reaching
the splash.

Root cause (measured, not inferred): NO termination and NO guest halt. The
`exec --ui` default path runs at the INFO log level (headless `check` runs
`--quiet` = WARN). During the boot idle-spin the scheduler has no Ready thread
for long stretches and `advance_to_next_wake_if_due` fires once per timed-wait
deadline-wake — hundreds of thousands of times. That `tracing::info!` emitted
~286K lines / 154 MB to disk in ~25s, throttling the guest so hard that the
instruction count crawled (deadline raced to 325M timebase while instructions
stayed near ~5M). The verbose run never terminated — it was alive and
logging-bound. Quiet `--ui` (no flood) reaches 161M instr / 2636 GPU draws in
~31s, exactly tracking headless.

Fix: demote the per-deadline-wake log from INFO to DEBUG (it is a hot-path
scheduler internal). Default `exec --ui` now emits ~1.6K log lines instead of
~235K over the same window; the idle-advance flood drops 286700 -> 0 at INFO,
and boot flows to the splash window. Log-level only: deterministic golden
byte-identical (n50m --stable-digest --expect matches, exit 0), 679 tests green.

Refutes the iterate-3O/3Q "8.8s --ui auto-close / BreakOuter" hypothesis (T1-T5
all negative): the cause was logging wall-clock cost, not a window/present
deadlock, watchdog, or guest exception.

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

crates/xenia-cpu/src/scheduler.rs

@@ -1293,7 +1293,15 @@ impl Scheduler {
         };
         t.quantum_remaining = QUANTUM_DEFAULT;
         self.recompute_slot_runnable(r.hw_id);
-        tracing::info!(
+        // DEBUG, not INFO: this fires once per timed-wait deadline-wake, which
+        // during the boot idle-spin happens hundreds of thousands of times. At
+        // INFO it floods the console/log file and throttles the interactive
+        // `exec --ui` path so hard (≈286K lines flushed to disk) that the guest
+        // crawls and never reaches the ~30–150M-instruction splash window —
+        // which masqueraded as a "--ui early termination" (iterate-3R). The
+        // headless `check` path runs `--quiet` (WARN) so it was never throttled.
+        // No execution-semantics change; deterministic golden is unaffected.
+        tracing::debug!(
             "scheduler: advanced to deadline {} waking hw={} idx={}",
             deadline,
             r.hw_id,