logger tests: join every writer future before its database closes, fix the gate-open race

The routing rule is global, so the agent it names lives next to it now
(~/.claude/agents/opus-coder.md), at medium effort and without the
milestone-spec wording that only fit this repository.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitea/workflows/gates.yml

@@ -101,13 +101,15 @@ jobs:
           sudo apt-get update -qq
           sudo apt-get install -qq -y --no-install-recommends qemu-user
 
-      # qemu 11.1.0 has a TCG regression: its translation-block optimization
-      # takes the panic branch of Zig's UBSan pointer-overflow check on a
-      # valid in-bounds pointer (11.0.3 and earlier are clean; bisected via
-      # the Arch archive). This image's qemu 8.2 is not affected — do not
-      # upgrade the emulator past 11.0.x until qemu fixes it.
+      # QEMU_ONE_INSN_PER_TB: qemu's TCG with translation-block optimization
+      # (observed on 8.2 and 11.1) takes the panic branch of Zig's UBSan
+      # pointer-overflow check on a valid in-bounds pointer (subs+b.cc is
+      # correct AArch64; real silicon and one-insn-per-tb both pass). The
+      # false abort makes qemu write a ~380 MB guest core per crashing test
+      # and the respawn loop ran the job into the runner's 3 h kill. ~10x
+      # slower, correct by construction.
       - name: Run test suite under qemu (plain suite, no -Dintegration)
-        run: zig build test-aarch64 -fqemu
+        run: QEMU_ONE_INSN_PER_TB=1 zig build test-aarch64 -fqemu
 
   frontend:
     # The light runner, a second act_runner at capacity 1 that advertises only