Thread last-request and cache usage fields through the exec event bridge (N3 WP6)

Why: squad consumes exec.ProcessEvent, which lacked the split usage
fields, so the WP6 accounting could not reach squad runs.
Evidence: GOWORK=off go vet green; additive fields on ProcessEvent.
Impact: additive only.
Refs: Agent-Native#35 #37, N3 plan WP6

adapters/native/backend.go

@@ -409,6 +409,10 @@ func (b *Backend) run(ctx context.Context, model string, budget *runtime.Budget,
 					Detail:               ev.Detail,
 					PromptTokens:         ev.PromptTokens,
 					CompletionTokens:     ev.CompletionTokens,
+					LastPromptTokens:     ev.LastPromptTokens,
+					LastCompletionTokens: ev.LastCompletionTokens,
+					CacheReadTokens:      ev.CacheReadTokens,
+					CacheWriteTokens:     ev.CacheWriteTokens,
 				})
 			}
 		},

exec/backend.go

@@ -57,6 +57,11 @@ type ProcessEvent struct {
 	// PromptTokens / CompletionTokens 在 Type=usage 时填写。
 	PromptTokens     int
 	CompletionTokens int
+	// 以下在 Type=usage 或 message_end 时填写（N3 WP6：上一请求与缓存命中拆分）。
+	LastPromptTokens     int
+	LastCompletionTokens int
+	CacheReadTokens      int
+	CacheWriteTokens     int
 }
 
 const (