Fix dash-incompatible slash substitution and bump omos size threshold

v1.14.31b made it through smoke-base and validate-base (reclaim worked),
but two narrow bugs blocked the rest:

1. 'Derive platform slug' in the per-arch matrix jobs used bash
   ${PLATFORM_PAIR//\//-} which dash (/bin/sh in the runner) can't
   parse — 'Bad substitution'. Rewrote with 'tr / -'.

2. smoke-omos image size 3107 MB tripped the 3000 MB guardrail. All
   functional checks pass; the mempalace-toolkit bake-in from v1.14.30b
   added ~100 MB and the threshold was stale. Bumped to 3200 MB.

No image-level changes.

scripts/smoke-test.sh

@@ -214,9 +214,12 @@ SIZE_BYTES=$(docker image inspect --format='{{.Size}}' "$IMAGE")
 SIZE_MB=$((SIZE_BYTES / 1024 / 1024))
 echo "  Uncompressed size: ${SIZE_MB} MB"
 
-# Thresholds (uncompressed): base 2500 MB, omos 3000 MB. Adjust as image content evolves.
+# Thresholds (uncompressed): base 2500 MB, omos 3200 MB. Adjust as image content evolves.
+# omos bumped 3000→3200 on v1.14.31c — mempalace-toolkit bake-in pushed the
+# omos variant to ~3.1 GB. Functional smoke checks all pass; this is a
+# guardrail, not a performance limit.
 THRESHOLD=2500
-[ "$VARIANT" = "omos" ] && THRESHOLD=3000
+[ "$VARIANT" = "omos" ] && THRESHOLD=3200
 if [ "$SIZE_MB" -gt "$THRESHOLD" ]; then
   fail "image size ${SIZE_MB} MB exceeds threshold ${THRESHOLD} MB for variant=$VARIANT"
 else