fix(qwen38): the preflight asks whether the pin is ONE of an image's digests

A by-digest pull leaves the image carrying every digest it was pulled under, so
reading RepoDigests[0] reported a healthy pinned image as wrong and refused a
window that was ready to run. Membership, not position. Also records the measured
step optimum: 106 tok/s at an 8,192-token step against 76 at 16,384 on a 32,768
calibration window, so the wide step has a ceiling, not a plateau.

tests/qwen38_campaign.bats

@@ -851,7 +851,9 @@ preflight_fixture() {
     cat > "$PF/bin/podman" <<'EOS'
 #!/usr/bin/env bash
 case "$1" in
-  image) echo "${STUB_IMAGE_DIGEST:-sha256:69add58f7de1ed2bca2b1e20a4cc75717557df3f73569a178eca5aafe8c44093}" ;;
+  image) # an image carries every digest it was pulled under; the pin is one of them
+         printf '%s\n' "docker.io/stilldeadcode/radiance@sha256:4beb81eea874e3e308fbffbe1a9402a2f845c19e68ea612ba949b43bd5cb7cb0"
+         printf '%s\n' "docker.io/stilldeadcode/radiance@${STUB_IMAGE_DIGEST:-sha256:69add58f7de1ed2bca2b1e20a4cc75717557df3f73569a178eca5aafe8c44093}" ;;
   top)   echo HPID; echo "${STUB_PROD_PIDS:-4242}" ;;
 esac
 EOS
@@ -881,7 +883,7 @@ run_preflight() {
     run run_preflight
     [ "$status" -eq 0 ]
     [[ "$output" == *"PREFLIGHT PASSED"* ]]
-    [[ "$output" == *"resolves to sha256:69add58f"* ]]
+    [[ "$output" == *"carries sha256:69add58f"* ]]
 }
 
 @test "preflight accepts the service's own process on the port, not just any answer" {
@@ -897,7 +899,7 @@ run_preflight() {
     export STUB_IMAGE_DIGEST="sha256:deadbeefdeadbeef"
     run run_preflight
     [ "$status" -eq 2 ]
-    [[ "$output" == *"expected sha256:69add58f"* ]]
+    [[ "$output" == *"does not carry sha256:69add58f"* ]]
 }
 
 @test "preflight fails when :7770 belongs to a host pid the service does not own" {