bench: REPS env var for run-all-langs.sh (vllm needs ≥4 to stabilise)

Per the post-mortem in 2026-05-08T01-15-02Z/MATRIX.md §7, vllm has
~10–20 % wire-byte variance across reps from non-deterministic
batching even at temperature=0. The 2-rep median is too few to
collapse that noise. Sglang + llama.cpp are deterministic and
stable at 2 reps.

Adds REPS env var (default 2) to run-all-langs.sh so the next
matrix rerun can pass REPS=4 just for vllm:

  REPS=2 packages/bench/scripts/run-all-langs.sh $RUN_ID sglang
  REPS=4 packages/bench/scripts/run-all-langs.sh $RUN_ID vllm
  REPS=2 packages/bench/scripts/run-all-langs.sh $RUN_ID llama.cpp

Author: wdunn001

packages/bench/scripts/run-all-langs.sh

@@ -50,54 +50,60 @@ mkdir -p "$OUTDIR"
 # shellcheck disable=SC2206
 SIZES_ARR=($SIZES)
 
-echo "=== run_id=$RUN_ID engine=$ENGINE sizes=$SIZES ==="
+# Reps default to 2 (sglang + llama.cpp are stable at 2). Override via env
+# for noisier engines — vllm at 2K tokens has ~10–20 % wire-byte variance
+# from non-deterministic batching/scheduling even at temperature=0; bumping
+# REPS=4 stabilises the median. See packages/bench/results/2026-05-08T01-15-02Z/MATRIX.md §7.
+REPS="${REPS:-2}"
+
+echo "=== run_id=$RUN_ID engine=$ENGINE sizes=$SIZES reps=$REPS ==="
 
 # 1. Python
 echo
 echo "--- python ---"
 .venv/bin/python -m codec_demo.matrix_run \
     --methodology "$METHODOLOGY" \
-    --sizes "${SIZES_ARR[@]}" --reps 2 \
+    --sizes "${SIZES_ARR[@]}" --reps "$REPS" \
     --out "$OUTDIR/python.json"
 
 # 2. TypeScript / Node
 echo
 echo "--- web ---"
 ( cd packages/demo && npx -y tsx src/matrix_run.ts \
     --methodology "../../$METHODOLOGY" \
-    --sizes "${SIZES_ARR[@]}" --reps 2 \
+    --sizes "${SIZES_ARR[@]}" --reps "$REPS" \
     --out "../../$OUTDIR/web.json" )
 
 # 3. .NET
 echo
 echo "--- dotnet ---"
 PATH="$HOME/.dotnet:$PATH" dotnet run --project packages/demo-dotnet -c Release --no-build -- \
     --methodology "$METHODOLOGY" \
-    --sizes "${SIZES_ARR[@]}" --reps 2 \
+    --sizes "${SIZES_ARR[@]}" --reps "$REPS" \
     --out "$OUTDIR/dotnet.json"
 
 # 4. Rust
 echo
 echo "--- rust ---"
 ./packages/demo-rust/target/release/codec-bench \
     --methodology "$METHODOLOGY" \
-    --sizes "${SIZES_ARR[@]}" --reps 2 \
+    --sizes "${SIZES_ARR[@]}" --reps "$REPS" \
     --out "$OUTDIR/rust.json"
 
 # 5. Java
 echo
 echo "--- java ---"
 PATH="$HOME/jdk/bin:$PATH" java -jar packages/demo-java/target/codec-bench.jar \
     --methodology "$METHODOLOGY" \
-    --sizes "${SIZES_ARR[@]}" --reps 2 \
+    --sizes "${SIZES_ARR[@]}" --reps "$REPS" \
     --out "$OUTDIR/java.json"
 
 # 6. C
 echo
 echo "--- c ---"
 ./packages/demo-c/build/codec-matrix \
     --methodology "$METHODOLOGY" \
-    --sizes "${SIZES_ARR[@]}" --reps 2 \
+    --sizes "${SIZES_ARR[@]}" --reps "$REPS" \
     --out "$OUTDIR/c.json"
 
 echo