backlog: inducer n=2, T74/T75 signature withdrawn, T68/T69 survives

docs/re/BACKLOG.md

@@ -783,6 +783,16 @@ search cannot find a *schedule*.
   been running; **T74 and T75 move off a semaphore onto an event** (the only
   threads that change what *kind* of object they wait for → chase these first);
   T50 drops `WaitMultiple(XEvent,XEvent)` → `Wait(XEvent)`.
+* 🔴 **(2026-08-25) WITHDRAWN: the T74/T75 signature.** It does **not** reproduce
+  — in run 2 they are on `Wait(XEvent)` while **healthy** and stay there. My
+  error: the healthy state is not fixed either, so a **one-sample-per-state**
+  diff cannot separate a freeze transition from ordinary variation between two
+  instants. ✅ **What survives two independent captures:** T68 and T69 go from
+  not-waiting to waiting in *both* runs (T69 on a semaphore, T68 on an event,
+  both times), and the bulk of threads are untouched (17/24 run 1, **21/24** run
+  2) ⇒ **not a whole-emulator stall** now has two captures behind it. **Next:
+  repeat the capture several times inside ONE healthy run** to learn which thread
+  states are stable before reading any frozen diff as meaningful.
 * ✅ **(2026-08-25) CORRECTION: `screen_id == flight` is NOT a freeze test.**
   The previous entry used it to rule out a freeze. `frozen.py` exists precisely
   because a frozen mission still classifies as `flight` (its docstring: 724 s of
@@ -794,7 +804,11 @@ search cannot find a *schedule*.
   Run flew **~670 s clean** with only the pilot; a heavy-CPU inducer
   (`heavy_read.py cpu`) started at 08:59:54 and it froze at **09:00:48 — 54 s
   later**, inside the 27–255 s band. Consistent with the existing tally but
-  **n=1, not causal** (confounder: elapsed mission time). ✅ **Refuted en route:
+  **n=1, not causal** (confounder: elapsed mission time). ⬆️ **UPGRADED to n=2
+  (2026-08-25)**: run 2 put the inducer **on from flight start** and froze
+  **~96 s** in (10:07:20 → 10:08:56). Both freezes sit inside the 27–255 s band;
+  the only long clean stretch is the only window with no inducer. Contrast is
+  sharp, confounder still untouched. ✅ **Refuted en route:
   the I/O was never the cost** — a full uncapped walk of every allocated extent is
   **371 MB in 0.1 s**, all page cache; the expense is Python-level CPU, ~4.2 s a
   pass. **Settle it with:** alternating inducer-on/off windows within a run,