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/config/runtimeconfig/channels_test.go

@@ -3,6 +3,7 @@ package runtimeconfig
 import (
 	"os"
 	"path/filepath"
+	"runtime"
 	"strings"
 	"testing"
 
@@ -176,8 +177,14 @@ func TestSaveChannelsRoundTrip(t *testing.T) {
 	if err != nil {
 		t.Fatalf("stat: %v", err)
 	}
-	if info.Mode().Perm()&0o077 != 0 {
-		t.Fatalf("channels.json mode = %v, want owner-only (0600)", info.Mode().Perm())
+	if runtime.GOOS != "windows" {
+		// Windows has no POSIX permission bits for regular files: Chmod only
+		// toggles the read-only flag and Stat reports 0666 regardless of the
+		// requested 0600, so this assertion is only meaningful on Unix-like
+		// platforms. SaveChannels still requests 0600 (see channels.go).
+		if info.Mode().Perm()&0o077 != 0 {
+			t.Fatalf("channels.json mode = %v, want owner-only (0600)", info.Mode().Perm())
+		}
 	}
 }
 

internal/config/runtimeconfig/plugins_test.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"os"
 	"path/filepath"
+	"runtime"
 	"strings"
 	"testing"
 )
@@ -39,8 +40,14 @@ func TestSaveLoadPlugins_RoundTrip(t *testing.T) {
 	if err != nil {
 		t.Fatalf("stat: %v", err)
 	}
-	if perm := info.Mode().Perm(); perm != 0o600 {
-		t.Fatalf("mode = %o, want 600", perm)
+	if runtime.GOOS != "windows" {
+		// Windows has no POSIX permission bits for regular files: Chmod only
+		// toggles the read-only flag and Stat reports 0666 regardless of the
+		// requested 0600, so this assertion is only meaningful on Unix-like
+		// platforms. SavePlugins still requests 0600 (see plugins.go).
+		if perm := info.Mode().Perm(); perm != 0o600 {
+			t.Fatalf("mode = %o, want 600", perm)
+		}
 	}
 	got, err := LoadPlugins(path)
 	if err != nil {