test(s023): the indicator test waits for the final message, not the init's idle

The postgres job (run 35534378835) read the init's idle and rendered
before the turn ("both sides are exactly equal"); the same race as
NOTES finding 9, on a slower pool. NOTES finding 10.

Signed-off-by: Ayla Croft <aylacroft@proton.me>

Author: HackTuah

slices/023-context-compaction/NOTES.md

@@ -133,3 +133,8 @@ $ mix credo --strict --all                                                     
   in a filtered view.
 - `Compactor` at 72 % coverage: the text-fallback branches are exercised live, not in the suite; a Mox
   provider test for the fallback is a small addition.
+
+10. **The postgres job caught a third test reading the init's idle** (run 35534378835: the indicator test, "both
+    sides are exactly equal"): the same race as finding 9, invisible on SQLite's timing and visible on the
+    slower pool. It waits for the final message now. Recorded as a pattern: after `live/2` mounts a session's
+    page, a test that waits on `{:state, :idle}` alone waits on nothing.