feat(aether-arena): benchmark-first scorer + witness chain + repeatability (M2/M5/M7)

Per direction "remove the initial number, optimize for benchmark first" + "include
witness chain capabilities for proof and repeatability analysis":

- Empty board, no seeded numbers: ledger seeds to genesis only. Every result is a
  real scoring-pipeline witness; RuView gets no hand-entered baseline.
- Real model scoring: aa_score_runner now loads predictions + an eval split
  (--split/--pred) and scores them through the real ruview_metrics pose harness —
  not just a synthetic fixture. Committed public smoke split (fixtures/smoke_*.json).
- Witness chain: each score emits a witness = inputs_sha256 (binds it to the exact
  inputs) + proof_sha256 (cross-platform-stable score hash) + harness_version.
- Repeatability analysis: --repeat N runs the harness N× and fails if it ever
  yields >=2 distinct proof hashes (16/16 identical locally).
- Witness ledger: ledger/ledger_tools.py — append-only, hash-chained, tamper-
  evident (seed/append/verify); editing any past row breaks the chain.
- CI gate extended: determinism + repeatability(16) + real-scoring smoke + ledger
  chain verify on every PR.

Co-Authored-By: claude-flow <ruv@ruv.net>

.github/workflows/aether-arena-harness.yml

@@ -55,17 +55,38 @@ jobs:
       - name: Run determinism gate
         run: cargo run -q -p wifi-densepose-train --bin aa_score_runner --no-default-features
 
-      # 3. Emit the score row into the PR run summary (leaderboard-ledger shape).
-      - name: Score row → job summary
+      # 3. Repeatability analysis (witness chain): the harness must produce one
+      #    identical proof hash across many runs — any nondeterminism fails here.
+      - name: Repeatability analysis (16 runs)
+        run: cargo run -q -p wifi-densepose-train --bin aa_score_runner --no-default-features -- --repeat 16
+
+      # 4. Real-scoring smoke: score a sample prediction against the public smoke
+      #    split, exercising the actual model-scoring path (not just the fixture).
+      - name: Real-scoring smoke test
+        run: |
+          cargo run -q -p wifi-densepose-train --bin aa_score_runner --no-default-features -- \
+            --split ../aether-arena/fixtures/smoke_split.json \
+            --pred  ../aether-arena/fixtures/smoke_pred.json --json
+
+      # 5. Witness ledger chain integrity: the append-only results ledger must
+      #    verify (every prev_hash link + row_hash intact = no silent edits).
+      - name: Verify witness ledger chain
+        working-directory: aether-arena/ledger
+        run: python3 ledger_tools.py verify
+
+      # 6. Emit the witness row + repeatability into the PR run summary.
+      - name: Witness row → job summary
         if: always()
         run: |
           ROW=$(cargo run -q -p wifi-densepose-train --bin aa_score_runner --no-default-features -- --json)
+          REP=$(cargo run -q -p wifi-densepose-train --bin aa_score_runner --no-default-features -- --repeat 16)
           {
-            echo "## AetherArena harness gate"
+            echo "## AetherArena harness gate (witness chain)"
             echo ""
-            echo "Deterministic score row (ADR-149 §2.2):"
+            echo "Deterministic witness (ADR-149 §2.2 / proof + repeatability):"
             echo '```json'
             echo "$ROW"
+            echo "$REP"
             echo '```'
             echo ""
             echo "If the determinism gate failed, the scoring maths changed: regenerate with"