ygg-verify: stop forcing CARGO_TARGET_DIR; parallel mutants shared it

cargo-mutants --jobs N workers all built into /src/target and tested each
other's binaries, so parallel kill counts were noise (idunn-watchdog Soul
s3). Cargo's default target dir is already inside /src.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

references/changelog.md

@@ -2,6 +2,25 @@
 
 Record each change to the skill together with the evidence that motivated it.
 
+## 2026-10-01: the stopgap no longer forces CARGO_TARGET_DIR
+
+In idunn-watchdog's Idunn incident cut, Soul pass s3 ran cargo-mutants twice
+on the same two mutants with `--jobs 3`, and the two runs gave opposite
+verdicts. The cause: `ygg-verify.sh` passed `-e CARGO_TARGET_DIR=/src/target`
+to every container. cargo-mutants gives each worker its own copy of the tree,
+but the forced variable points them all at one target directory, so a worker
+can test another worker's binary. With the variable unset, `true` was reported
+missed and `false` caught, which is the right answer.
+
+Every `--jobs` greater than 1 mutation count reported through the stopgap
+before this change is unreliable. That covers both sessions' campaigns. A
+survivor may have been reported caught, or a kill reported missed. Treat those
+counts as unproven, and rerun a cut's mutation evidence when its rule matters.
+
+What changed: the variable is gone. Cargo's default, `<workspace>/target`, is
+already inside `/src`. The scar about not sharing build output between
+checkouts still stands, and cargo's default honours it.
+
 ## 2026-09-30: Soul pays for each rerun once
 
 The sccache measurement below showed that verify-slot queueing dominates a

tools/stopgap/ygg-verify.sh

@@ -22,6 +22,10 @@
 #   3. Runs <command> in <image> under a CPU and memory cap, niced. Each
 #      toolchain gets one shared registry cache. The build output directory
 #      stays inside the work directory and is never shared between checkouts.
+#      CARGO_TARGET_DIR is deliberately NOT set: cargo's default (<workspace>/
+#      target) is already inside /src, and forcing it made cargo-mutants' --jobs
+#      workers share one target dir and test each other's binaries, so parallel
+#      kill counts were noise (Soul, idunn-watchdog, 2026-10-01).
 #   4. Removes the work directory unless KEEP=1, and exits with the job's status.
 #
 # At most $SLOTS jobs run at once on Yggdrasil. The operator raised this to 3 on
@@ -190,7 +194,7 @@ sudo nice -n 10 docker run --rm --name "$cname" --cpus="$cpus" --memory="$mem" \
   -v "$work:/src" -v /etc/machine-id:/etc/machine-id:ro \
   -v eureka-cargo-registry:/usr/local/cargo/registry -v eureka-nuget:/root/.nuget/packages \
   -v eureka-sccache:/sccache \
-  -e CARGO_TARGET_DIR=/src/target -e BASH_ENV=/tmp/.ygg-bash-env \
+  -e BASH_ENV=/tmp/.ygg-bash-env \
   -e GIT_CONFIG_COUNT=1 -e GIT_CONFIG_KEY_0=safe.directory -e GIT_CONFIG_VALUE_0='*' \
   $docker_args \
   -w /src "$image" bash -o pipefail -c "printf 'set -o pipefail\n' >/tmp/.ygg-bash-env; $cmd"