Route conversations through a squad entry when selected

Why: Squad steps previously ran without tools, and the product had no way to make the squad the conversation entry without losing streaming, cards, or the deliverable guard.
Evidence: go test ./... -count=1 passed, including streaming, cardflow, false-deliverable rejection, tool allowlist, model routing, usage, cancel, and turn and tool budgets.
Impact: runtime.entry defaults to direct. squad uses one kernel step for simple work and the leader for the rest. squad_orchestrate is hidden in that mode. The default is not switched.
Refs: deliverable-quality-system B3-B5
Co-authored-by: Cursor <cursoragent@cursor.com>

eval/README.md

@@ -7,3 +7,5 @@
 ```
 cd modules/backend && go test ./eval/ ./internal/quality/ -count=1
 ```
+
+两种入口的切换由 `DecideSwitch` 判断：squad 分数不低于 direct，并且直通路径多出来的首字延迟 P50 不超过 3 秒。没有同一评测集上的实测分数时，结果是不切换，`runtime.entry` 默认保持 `direct`。

eval/switch.go

@@ -0,0 +1,45 @@
+package eval
+
+import (
+	"fmt"
+	"time"
+)
+
+// EntryScore 是一种入口在同一评测集上的结果。
+type EntryScore struct {
+	Name          string
+	Score         int
+	FirstTokenP50 time.Duration
+}
+
+// SwitchDecision 判断能不能把默认入口改成 squad。
+// 两条都要满足：squad 分数不低于 direct，直通路径多出来的首字延迟 P50 不超过 3 秒。
+type SwitchDecision struct {
+	Met    bool
+	Reason string
+}
+
+func DecideSwitch(direct, squad *EntryScore) SwitchDecision {
+	if direct == nil || squad == nil {
+		return SwitchDecision{Met: false, Reason: "还没有两种入口在同一评测集上的实测分数，默认保持 direct"}
+	}
+	extra := squad.FirstTokenP50 - direct.FirstTokenP50
+	if extra < 0 {
+		extra = 0
+	}
+	if squad.Score < direct.Score {
+		return SwitchDecision{Met: false, Reason: fmt.Sprintf("squad 分数 %d 低于 direct %d", squad.Score, direct.Score)}
+	}
+	if extra > 3*time.Second {
+		return SwitchDecision{Met: false, Reason: fmt.Sprintf("直通路径额外首字延迟 %s，超过 3 秒", extra)}
+	}
+	return SwitchDecision{Met: true, Reason: fmt.Sprintf("squad %d 不低于 direct %d，额外首字延迟 %s", squad.Score, direct.Score, extra)}
+}
+
+// Report 写成可以附在切换 PR 上的短报告。未达标时明确保持 direct。
+func Report(dec SwitchDecision) string {
+	if dec.Met {
+		return "切换条件已满足：" + dec.Reason + "。可以把 runtime.entry 的默认值改为 squad，并保留 direct 作为回退。合入前需要产品确认。"
+	}
+	return "切换条件未满足：" + dec.Reason + "。runtime.entry 默认值保持 direct。"
+}

eval/switch_test.go

@@ -0,0 +1,27 @@
+package eval
+
+import (
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestSwitchStaysDirectWithoutScores(t *testing.T) {
+	dec := DecideSwitch(nil, nil)
+	if dec.Met || !strings.Contains(Report(dec), "保持 direct") {
+		t.Fatalf("%+v %s", dec, Report(dec))
+	}
+}
+
+func TestSwitchRequiresScoreAndLatency(t *testing.T) {
+	direct := &EntryScore{Name: "direct", Score: 80, FirstTokenP50: time.Second}
+	slow := &EntryScore{Name: "squad", Score: 90, FirstTokenP50: 5 * time.Second}
+	if DecideSwitch(direct, slow).Met {
+		t.Fatal("extra latency above 3s must block the switch")
+	}
+	ok := &EntryScore{Name: "squad", Score: 80, FirstTokenP50: 3 * time.Second}
+	dec := DecideSwitch(direct, ok)
+	if !dec.Met || !strings.Contains(Report(dec), "改为 squad") {
+		t.Fatalf("%+v %s", dec, Report(dec))
+	}
+}