bench.sh: simplify per code-review findings

- Drop NPROMPTS=5 from the usage docstring — the variable was never
read; the example was a lie.
- Extract the duplicated 'jq tok/s math' expression into a tok_per_s()
helper. Two callers, identical formula — one place to fix if the
rounding ever needs to change.
- Route red() error output to stderr instead of stdout. Callers parsing
the table off stdout shouldn't see error noise mixed in.
- Cache the /api/tags response into TAGS once and reuse for both the
reachability check and the model-present jq filter, instead of
hitting the endpoint twice back-to-back.

Behavior unchanged on the happy path; failure messages now redirect
to stderr (verified with MODEL=nonexistent-model).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

scripts/bench.sh

@@ -8,7 +8,7 @@
 #
 # Usage:
 #   ./scripts/bench.sh                       # uses MODEL=janus-27b
-#   MODEL=janus-27b NPROMPTS=5 ./scripts/bench.sh
+#   MODEL=janus-27b ./scripts/bench.sh
 #   HOST=http://localhost:11434 ./scripts/bench.sh
 #
 # Requires: curl, jq, a running Ollama daemon with the model created.
@@ -17,21 +17,27 @@ set -euo pipefail
 MODEL="${MODEL:-janus-27b}"
 HOST="${HOST:-http://localhost:11434}"
 
-red()   { printf "\033[31m%s\033[0m\n" "$*"; }
+red()   { printf "\033[31m%s\033[0m\n" "$*" >&2; }
 green() { printf "\033[32m%s\033[0m\n" "$*"; }
 blue()  { printf "\033[34m%s\033[0m\n" "$*"; }
 
+# tok_per_s <eval_count> <eval_duration_ns> -> "X.YZ" (2 dp, floor).
+tok_per_s() {
+    jq -n --argjson c "$1" --argjson n "$2" '($c / ($n / 1e9)) | . * 100 | floor / 100'
+}
+
 for dep in curl jq; do
     if ! command -v "$dep" >/dev/null 2>&1; then
         red "[!] missing dependency: $dep"; exit 1
     fi
 done
 
-if ! curl -fsS "${HOST}/api/tags" >/dev/null; then
+# Single /api/tags fetch covers both checks below.
+if ! TAGS="$(curl -fsS "${HOST}/api/tags")"; then
     red "[!] Ollama not reachable at ${HOST}"
     exit 1
 fi
-if ! curl -fsS "${HOST}/api/tags" | jq -e --arg m "${MODEL}" '.models[] | select(.name | startswith($m))' >/dev/null; then
+if ! jq -e --arg m "${MODEL}" '.models[] | select(.name | startswith($m))' >/dev/null <<<"${TAGS}"; then
     red "[!] Model '${MODEL}' not found. Build it first: ./scripts/build.sh"
     exit 1
 fi
@@ -86,7 +92,7 @@ for i in "${!PROMPTS[@]}"; do
     fi
 
     eval_ms=$(( eval_ns / 1000000 ))
-    toks_per_s="$(jq -n --argjson c "$eval_count" --argjson n "$eval_ns" '($c / ($n / 1000000000)) | . * 100 | floor / 100')"
+    toks_per_s="$(tok_per_s "$eval_count" "$eval_ns")"
     printf "%-4s  %8s  %12s  %8s\n" "$((i+1))" "$eval_count" "$eval_ms" "$toks_per_s"
 
     TOTAL_TOKENS=$(( TOTAL_TOKENS + eval_count ))
@@ -94,5 +100,5 @@ for i in "${!PROMPTS[@]}"; do
 done
 
 echo
-avg="$(jq -n --argjson c "$TOTAL_TOKENS" --argjson n "$TOTAL_NS" '($c / ($n / 1000000000)) | . * 100 | floor / 100')"
+avg="$(tok_per_s "$TOTAL_TOKENS" "$TOTAL_NS")"
 green "[+] aggregate: ${TOTAL_TOKENS} tokens / $(( TOTAL_NS / 1000000 )) ms = ${avg} tok/s"