test(platform): 修 Windows 上平台脆弱的断言（不削弱测试意图）

- trace/task: 亚 tick 耗时合法读到 0（本机时钟步长约 0.44ms），改为断言非负且 Duration == EndedAt-StartedAt
- user/runtimeconfig: Windows 无法表示普通文件的 POSIX 权限位(Stat 恒 0666)，0600 断言仅在非 Windows 生效，注释写明实现确实请求 0600
- exec: MSYS pwd 把同一目录渲染成 /tmp/...；改用「标记文件相对名解析」+「与显式 root 的 pwd 输出逐字相等」两条跨平台断言 + 变异验证
- 只读排查全仓同类脆弱断言，结果记入 DevDocs/工作小结

internal/runtime/trace/trace_test.go

@@ -72,8 +72,15 @@ func TestSpan_Lifecycle(t *testing.T) {
 	if sp.EndedAt.Before(before) {
 		t.Fatalf("EndedAt %v is before snapshot %v", sp.EndedAt, before)
 	}
-	if sp.Duration <= 0 {
-		t.Fatalf("Duration = %v, want > 0", sp.Duration)
+	// The monotonic clock may be too coarse to tick between StartedAt and
+	// EndedAt, so a sub-tick span legitimately reads Duration == 0. Assert the
+	// platform-independent invariant instead: Duration is non-negative and is
+	// exactly the delta of the two stamps.
+	if sp.Duration < 0 {
+		t.Fatalf("Duration = %v, want >= 0", sp.Duration)
+	}
+	if want := sp.EndedAt.Sub(sp.StartedAt); sp.Duration != want {
+		t.Fatalf("Duration = %v, want EndedAt-StartedAt = %v", sp.Duration, want)
 	}
 	if len(sp.Events) != 1 || sp.Events[0].Type != "token" {
 		t.Fatalf("Events = %+v, want one 'token' event", sp.Events)
@@ -241,8 +248,13 @@ func TestJSONRecorder_Output(t *testing.T) {
 	if childRec.Status != SpanStatusOK {
 		t.Fatalf("childRec.Status = %q, want ok", childRec.Status)
 	}
-	if childRec.Duration <= 0 {
-		t.Fatalf("childRec.Duration = %v, want > 0", childRec.Duration)
+	// As in TestSpan_Lifecycle: a sub-tick span reads Duration == 0 on platforms
+	// with a coarse monotonic clock, so assert the recorded delta invariant.
+	if childRec.Duration < 0 {
+		t.Fatalf("childRec.Duration = %v, want >= 0", childRec.Duration)
+	}
+	if want := childRec.EndedAt.Sub(childRec.StartedAt); childRec.Duration != want {
+		t.Fatalf("childRec.Duration = %v, want EndedAt-StartedAt = %v", childRec.Duration, want)
 	}
 	if childRec.Props["model"] != "gpt-test" {
 		t.Fatalf("childRec.Props = %+v, want model key", childRec.Props)