Carry the split usage fields through squad events (N3 WP6)

Why: the exec bridge now threads Last*/cache (Agent-Native#38); squad
ProcessEvent carries them to hosts (the product's squad runner reads
them for usage accounting).
Evidence: GOWORK=off go vet green across squad and adapters/native;
nested pin at the merged native.
Impact: additive event fields.
Refs: Agent-Native#38, N3 plan WP6

adapters/native/register.go

@@ -138,6 +138,8 @@ func toNative(req squad.ExecRequest) exec.Request {
 			prev(squad.ProcessEvent{
 				Type: ev.Type, Text: ev.Text, Tool: ev.Tool, Detail: ev.Detail,
 				PromptTokens: ev.PromptTokens, CompletionTokens: ev.CompletionTokens,
+				LastPromptTokens: ev.LastPromptTokens, LastCompletionTokens: ev.LastCompletionTokens,
+				CacheReadTokens: ev.CacheReadTokens, CacheWriteTokens: ev.CacheWriteTokens,
 			})
 		}
 	}

squad/port.go

@@ -64,12 +64,17 @@ type ExecBudget struct {
 
 // ProcessEvent 是过程帧。
 type ProcessEvent struct {
-	Type             string
-	Text             string
-	Tool             string
-	Detail           string
-	PromptTokens     int
-	CompletionTokens int
+	Type                 string
+	Text                 string
+	Tool                 string
+	Detail               string
+	PromptTokens         int
+	CompletionTokens     int
+	// 以下在 Type=usage 时填写（N3 WP6：上一请求与缓存命中拆分）。
+	LastPromptTokens     int
+	LastCompletionTokens int
+	CacheReadTokens      int
+	CacheWriteTokens     int
 }
 
 // ExecResult 是一次步骤执行的终态。