feat(serial): Step 4 事件推送替代轮询（RFC #3 收官）

[Gyanano]

RFC #3 Step 4：FrameBus + Nagle 合批 + serial://frames 桥接（收官步）

跟踪 issue：#3（前置：#4、#5、#6、#10）

后端

• bus.rs 新增：Frame {session, seq, dir, t_mono_ns, t_wall, Arc<[u8]>}——seq 会话内从 1 严格递增、TX/RX 同序列；FrameBus 有界订阅队列（满则 drop-oldest + dropped_before 前缀缺口计数，读线程零阻塞）；Nagle 合批（GUI_DEFAULT 16ms/64KiB/512 帧/4096 队列；空闲消费者单帧 ≥4ms 间隔即推）
• 读循环与 send_data 在写缓冲之后 publish（双写存续期：日志缓冲/录制/统计不动，全部 golden 测试原样通过）
• get_logs_snapshot {epoch, session, entries} 初始对齐；clear_logs 返回递增 epoch 防快照复活

桥接（main.rs）

应用级 pump 线程：有界阻塞 recv → 显示装饰（display_text/时间戳）→ emit("serial://frames")；热路径 DTO 不携带原始字节（seq/dir/len + 装饰文本，JSON 体积大幅下降）

前端

• useSerialLogs：快照对齐 → 增量 append（rAF 节流）；seq 去重；session 变更自动重对齐；epoch 不匹配的快照直接丢弃（清空永不复活）；dropped_before>0 插入「丢失 N 帧」占位行；trim 镜像后端 1000 上限；LogViewer 行键改 session-seq
• 回滚开关：localStorage serialEventPush=0 恢复 100ms 轮询

验证

• cargo test 40/40（bus 5 个单测：seq 单调/会话重置/单帧即推/合批/drop-oldest 缺口对账；harness 集成测试：真实读线程 TX+RX seq 连续无丢无重）
• 有头应用 + FTDI 环回人工验证：收发即时性、狂发计数无丢无重、清空不复活、搜索/滚底/计数器、拔线检测全部通过

后续解锁

• bug(serial): 高负载持续接收时驱动层疑似丢字节（读线程被轮询克隆饿死） #9（高负载丢字节）：轮询克隆饿死读线程的病因已除，待复测关闭
• feat(serial): TX write-behind 发送队列 + 背压与失败上报 #7（TX 队列）：事件通道就绪，可开工

---

Note

Medium Risk
Changes the serial read/send hot path, background pump thread, and frontend log state machine; rollback polling exists but default behavior is new event synchronization logic.

Overview
Replaces the default 100ms full-clone get_logs polling with an event-driven log path: initial get_logs_snapshot alignment plus incremental serial://frames batches from a backend pump thread.

Backend adds a FrameBus (bus.rs) with per-session seq (TX/RX share one sequence), bounded subscriber queues (drop-oldest + dropped_before), and Nagle-style batching. RX and TX still write the in-memory log buffer first, then publish to the bus. clear_logs now returns a monotonic epoch; snapshots carry epoch/session so clears cannot resurrect stale rows. New command get_logs_snapshot.

Frontend useSerialLogs listens for batches, dedupes by seq, resyncs on session change, rAF-throttles appends, shows overload placeholders when dropped_before > 0, and mirrors the ~1000 entry cap. localStorage serialEventPush=0 restores polling. LogViewer row keys use session-seq / gap_key.

Existing get_logs remains for rollback/export alignment during the dual-write period.

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