ci: bump a2ml-validate-action to 6bff6ec (s-expression form support)

Validator PR #26 (merged 2026-06-01) taught the action to recognise
both TOML `name = "x"` AND s-expression `(name "x")` identity
and version forms.

Estate sweep manual-retry: the throttled sweep at /tmp/a2ml-sha-sweep/
hit a transient clone error on aerie at 04:23:30Z; sweep already
moved past. Manually retried 2026-06-02 ~06:00Z.

Real-world bite: panic-attack#94. Wave: 215-repo estate sweep.

Author: hyperpolymath

.github/workflows/dogfood-gate.yml

@@ -38,7 +38,7 @@ jobs:
 
       - name: Validate A2ML manifests
         if: steps.detect.outputs.count > 0
-        uses: hyperpolymath/a2ml-validate-action@59145c7d1039fa3059b3ecacdb50ee23d7505898 # main
+        uses: hyperpolymath/a2ml-validate-action@6bff6ec134fc977e86d25166a5c522ddea5c1e78 # main
         with:
           path: '.'
           strict: 'false'