feat(loop): Top5#5 上下文溢出压缩 + 每轮 assemble（对齐 OpenClaw context-engine）——Top5 全完成

- 每轮 assemble(transformContext 等价)：run.go 每 step 前 assembleContext——依次 Inject(每轮
  重新 Retriever.Inject 不缓存首轮)、WorkingMemory 前置 Summarize、可插拔 TransformContext 投影/
  过滤；生成的 view 交 step，msgs 保持纯对话供工具结果追加，记忆不重复注入、不写回 checkpoint。
- Compactor：结构分支感知压缩——estimateContextTokens 启发式超 MaxContextTokens 且消息≥3 才动；
  findKeepStart 按 user 消息切整轮从后往前累加(至少保留最后一整轮→尾随工具结果绝不切片/不跨轮)；
  旧轮 user/assistant 对压成一条 RoleSystem 摘要置顶，既有摘要前缀识别合并防叠；capSummary 上限
  4000 rune；ctx 即 abortSignal(取消返回 ctx.Err，压缩失败仅记日志不中断 run)。
- 选项：Options 增 MaxContextTokens/Compactor/TransformContext，均 nil-tolerant(默认=现有行为，
  零值不压缩)；DefaultMaxContextTokens=32768。Compactor 接口预留注入 LLM 摘要。

go build/vet/test -count=1 全绿 + windows/darwin 交叉编译。12 断言覆盖尾随工具结果保留/不跨轮切割/
摘要上限/摘要合并/ctx 取消/每轮重检索/TransformContext 逐轮/超限压缩/默认关闭。-race 受宿主
TSan VMA 限制无法运行(与代码无关)。

**至此 Top5(1~5) 全部完成并推库。**

internal/runtime/loop/loop.go

@@ -12,6 +12,7 @@
 package loop
 
 import (
+	"context"
 	"errors"
 	"io"
 	"log/slog"
@@ -91,9 +92,12 @@ type Loop struct {
 	maxRounds       int
 	toolExec        string
 	toolParallelism int
-	hooks           ToolLoopHooks
-	onStream        func(model.StreamEvent)
-	log             *slog.Logger
+	hooks              ToolLoopHooks
+	onStream           func(model.StreamEvent)
+	maxContextTokens   int
+	compactor          Compactor
+	transformContext   func(context.Context, []model.Message, int) []model.Message
+	log                *slog.Logger
 }
 
 // New builds a Loop from opts, applying defaults. A non-positive MaxRounds
@@ -114,23 +118,30 @@ func New(opts Options) *Loop {
 	if toolParallelism <= 0 {
 		toolParallelism = DefaultToolParallelism
 	}
+	compactor := opts.Compactor
+	if compactor == nil {
+		compactor = defaultCompactor{}
+	}
 	return &Loop{
-		agent:           opts.Agent,
-		tools:           opts.Tools,
-		memory:          opts.Memory,
-		retriever:       opts.Retriever,
-		working:         opts.WorkingMemory,
-		extractor:       opts.Extractor,
-		checkpoint:      opts.Checkpoint,
-		tracer:          opts.Tracer,
-		guards:          opts.Guards,
-		personaID:       opts.PersonaID,
-		maxRounds:       maxRounds,
-		toolExec:        toolExec,
-		toolParallelism: toolParallelism,
-		hooks:           opts.Hooks,
-		onStream:        opts.OnStream,
-		log:             sourcedLog(opts.Log),
+		agent:            opts.Agent,
+		tools:            opts.Tools,
+		memory:           opts.Memory,
+		retriever:        opts.Retriever,
+		working:          opts.WorkingMemory,
+		extractor:        opts.Extractor,
+		checkpoint:       opts.Checkpoint,
+		tracer:           opts.Tracer,
+		guards:           opts.Guards,
+		personaID:        opts.PersonaID,
+		maxRounds:        maxRounds,
+		toolExec:         toolExec,
+		toolParallelism:  toolParallelism,
+		hooks:            opts.Hooks,
+		onStream:         opts.OnStream,
+		maxContextTokens: opts.MaxContextTokens,
+		compactor:        compactor,
+		transformContext: opts.TransformContext,
+		log:              sourcedLog(opts.Log),
 	}
 }