sweep up scrum WARNs — model const, stale config, temp_path entropy, smoke gate

Four findings deferred from the 2026-05-02 scrum, all 1-5 line fixes:

W1 (kimi WARN @ scrum_master_pipeline.ts:1143) — `gemini-3-flash-preview`
hardcoded twice in MAP and REDUCE phases. Extracted TREE_SPLIT_MODEL +
TREE_SPLIT_PROVIDER constants near the existing config block. Diverging
the two would break tree-split coherence (per-shard digests must come
from the same model the reducer collapses).

W2 (qwen WARN @ providers.toml:30) — stale `kimi-k2:1t` reference in
operator-facing comments after PR #13 noted it's upstream-broken. Reframed
as historical context ("was X here pre-2026-05-03 — that model is broken")
so future operators don't paste-route from the comment.

W3 (opus WARN @ vectord-lance/src/lib.rs:622) — temp_path() entropy was
only pid+nanos, which collide under tokio scheduling when multiple tests
in the same cargo process create temp dirs back-to-back. Added per-process
AtomicU64 sequence counter — guarantees uniqueness regardless of clock.

W4 (opus INFO @ scripts/lance_smoke.sh:38) — `|| echo '{}'` swallowed
curl transport failures (gateway down, network broken, timeout), surfacing
as misleading "no method field" jq errors at the next probe. Now captures
$? separately, gates a "curl reachable" probe, and only falls back to
empty body for the dependent jq parse. Smoke went 9 → 10 probes.

Verified: vectord-lance 7/7 tests PASS, gateway cargo check clean,
lance_smoke.sh 10/10 PASS against live gateway.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/lance_smoke.sh

@@ -33,9 +33,18 @@ PROBE "gateway /v1/health responds" \
   bash -c "curl -sf -m 3 $GATEWAY/v1/health -o /dev/null"
 
 # ── 1. Search returns IVF_PQ results on existing dataset ────────
+# Capture curl status separately so a transport-level failure (gateway
+# down, network broken, timeout) shows up as its own probe — instead of
+# being swallowed by `|| echo '{}'` which would surface as the next jq
+# probe failing with a misleading "no method field" message. Per opus
+# INFO at lance_smoke.sh:38 from the 2026-05-02 scrum.
 RESP=$(curl -sS -m 30 -X POST "$PREFIX/search/$DATASET" \
   -H 'Content-Type: application/json' \
-  -d '{"query":"forklift operator","top_k":3}' 2>/dev/null || echo '{}')
+  -d '{"query":"forklift operator","top_k":3}' 2>/dev/null)
+CURL_RC=$?
+PROBE "search/$DATASET curl reachable (exit 0)" \
+  test "$CURL_RC" = "0"
+[ "$CURL_RC" != "0" ] && RESP='{}'
 PROBE "search/$DATASET returns top-3 lance_ivf_pq results" \
   bash -c "echo '$RESP' | jq -e '.method == \"lance_ivf_pq\" and (.results | length) == 3' >/dev/null"