feat(loop,gateway): 事件批 — tool_execution 三事件 + streamId + turn.start/end（裁决 01.4.1/01.4.2/01.4.3/01.4.5/03.4.4/1.12）

执行方案阶段 2 · P2-3（`DevDocs/audit/_plan.md`）。契约 `Docs/api-client.md` → **v0.50**。

- **01.4.1 `tool_execution` 拆三事件**：`model/stream.go` 删旧 `StreamKindToolExecution`，新增
  `tool_execution.start/update/end` + `ToolPhaseRunning/Done/Error`（与 design/19 status 同值）；
  `StreamEvent` 增 `Tool`/`Phase`；`loop/tools.go` 的 `emitToolExecution` 重写为结构化字段、
  **Delta 恒空**（断言锁死），`ID` 复用 provider tool call id（可与 `tool_call.*` 关联）。
  ⚠️ **诚实边界**：start/end 有真实生产者；`update` 的 wire 映射已实现并单测锁定，但**工具分发器
  无中途进度上报源 ⇒ loop 暂不产出 update 帧**（已在 api-client §3.1 如实登记）。
- **03.4.4 phase 驱动图节点状态**：`invokeOne` 用**单一 phase 决策**同时决定 end 帧 phase、
  span 的 `phase` prop 与终态；`trace/graph.go` 新增 `FlowStatusForPhase`，`graph_build.go` 的
  `AddSpan` 对 tool 节点**优先消费 phase prop**（缺省回落 span 终态 ⇒ 两路不可能矛盾）。
- **01.4.2 `streamId`**：`StreamEvent` 增 `StreamID`；`loop/stream.go` 每次 ChatStream 生成
  `stream-<hex>`（crypto/rand，降级时间戳），该流所有帧共享、多步工具轮取新 id；`ws_bridge`
  把它并进**模型流帧** `data`（`tool_execution.*`/`turn.*` 不带）。语义区分：`turnId` = 哪轮回答，
  `streamId` = 这条流式消息。
- **01.4.3 `turn.start`/`turn.end`**：`loop/run.go` 入口发 start、defer 发 end，**覆盖全部出口**
  （最终答案/安全网终止/预算耗尽/步骤错误）；帧**无 data 载荷**；静音的内部 run（backthink/子代理）
  不发。顺带把 91 行的 `Run` 拆成 `driveTurn` + `settleAndReturn`（清掉一处既有 >80 行违规）。
- **01.4.5**：仅 `Docs/api-client.md` §3.5 补 `graph_update` 三范围（`web`/`web:<uuid>`/`sess-backthink`），
  与 `wire_runtime.go` 的 allow 谓词一致，**零代码变更**。

验证：`go build ./...` 绿；本批四包（loop/model/trace/gateway）vet + 测试绿（含 `-count=1`）；
linux/amd64 + darwin/arm64 交叉编译绿；本批 16 文件 ≤500 行、函数 ≤80 行。
**真 WS 帧断言 8 条**：帧序 `turn.start → message.delta → tool_execution.start →
tool_execution.end → message.delta → turn.end`（+`stream_end`）、turn 边界无载荷且整轮 turnId 一致、
两条模型流 `streamId` 互异、tool 帧 `{tool,phase,id}` 且无 `delta`/`streamId` 键、
`turn.end` 在步骤出错时也发出、内部静音 run 不发边界帧。
**变异验证**：把 `ev.StreamID = streamID` 改为赋空 → 两条断言 FAIL（`streamId` 为空/两流同空），
还原后复绿（直接删行会先被编译期 `declared and not used` 拦截）。

未验证：前端未消费这些帧（本轮未动 `frontend/**`，「前端能渲染」待前端批）；无真实模型端到端。

internal/runtime/model/stream.go

@@ -33,11 +33,40 @@ const (
 	// StreamKindToolCallEnd closes a tool invocation block.
 	StreamKindToolCallEnd StreamKind = "tool_call_end"
 
-	// StreamKindToolExecution reports intermediate progress while the loop runs
-	// a tool that the model requested (design/04 §1.1 tool_execution_update). It
-	// is emitted by the tool loop, not the model adapter; reserved for a later
-	// milestone, but the wire mapping is defined so the frontend can react.
-	StreamKindToolExecution StreamKind = "tool_execution_update"
+	// StreamKindToolExecutionStart opens the execution of a tool the model
+	// requested (design/04 §1.1: entering execution). It is emitted by the tool
+	// loop, not the model adapter; Phase carries the running label.
+	StreamKindToolExecutionStart StreamKind = "tool_execution_start"
+	// StreamKindToolExecutionUpdate reports intermediate progress or partial
+	// output while a requested tool runs (design/04 §1.1). Multiple frames per
+	// execution are allowed; the wire mapping is defined, and a producer appears
+	// once tools can report mid-execution progress.
+	StreamKindToolExecutionUpdate StreamKind = "tool_execution_update"
+	// StreamKindToolExecutionEnd closes a tool execution with its terminal
+	// phase (done or error). It is emitted by the tool loop after Invoke
+	// returns.
+	StreamKindToolExecutionEnd StreamKind = "tool_execution_end"
+
+	// StreamKindTurnStart opens an assistant turn at the loop's run boundary
+	// (design/04 §1.1 turn.start). It carries no payload: the frame shell's
+	// session/turn identity brackets the turn on the client.
+	StreamKindTurnStart StreamKind = "turn_start"
+	// StreamKindTurnEnd closes an assistant turn at the loop's run boundary,
+	// however the run settled (final answer, guardrail stop, terminated tool
+	// loop, exhausted rounds or an error).
+	StreamKindTurnEnd StreamKind = "turn_end"
+)
+
+// 工具执行 phase 标签（design/04 §1.1 细粒度 phase）。取值与 design/19 §1
+// 的节点 status 枚举（running/done/error）一致：start/update 帧带 running，
+// end 帧带 done 或 error，前端与实时流程图据此驱动工具节点的状态。
+const (
+	// ToolPhaseRunning 标记工具进入执行或仍在执行中。
+	ToolPhaseRunning = "running"
+	// ToolPhaseDone 标记工具执行成功结束。
+	ToolPhaseDone = "done"
+	// ToolPhaseError 标记工具执行失败结束。
+	ToolPhaseError = "error"
 )
 
 // StreamEvent is a single structured event in a streamed completion. It replaces
@@ -49,14 +78,30 @@ type StreamEvent struct {
 	Kind StreamKind
 	// Delta carries the incremental payload: body text for TextDelta, reasoning
 	// text for ThinkingDelta, or a partial_argument fragment for ToolCallDelta.
+	// Tool-execution events never pack their phase or tool name in here —
+	// those live in the structured Phase/Tool fields.
 	Delta string
 	// Index is the zero-based content-block index the event belongs to, when the
 	// provider reports one. The frontend uses it to correlate a segment's
 	// start/delta/end events.
 	Index int
 	// ID is the provider block/index identifier for a tool call (e.g. Anthropic's
-	// tool_use block id). Empty for text segments.
+	// tool_use block id). Empty for text segments. Tool-execution events reuse it
+	// so the client can correlate an execution with its tool_call.* frames.
 	ID string
+	// Tool is the invoked tool's name on a tool_execution.* event. Empty on
+	// every other kind.
+	Tool string
+	// Phase is the fine-grained execution label (ToolPhase*) on a
+	// tool_execution.* event: running on start/update, done or error on end.
+	// Empty on every other kind.
+	Phase string
+	// StreamID identifies the single model stream (one ChatStream call) a frame
+	// belongs to. The loop stamps it at the stream boundary so every frame of
+	// the same streamed message shares it, while a multi-step turn's later
+	// model calls get fresh ids; consumers distinguish it from TurnID, which
+	// stays constant across the whole turn.
+	StreamID string
 	// SessionID is the conversation the streamed event belongs to. It is stamped
 	// by the loop at the turn boundary (loop.streamStep), which reads the session
 	// the Run was started for; a model adapter, running below the loop and with no