fix(can): Eliminate persistent CAN RX latency after startup bursts

[qzhhhi]

Drain all pending CAN frames in each RX interrupt on c_board and rmcs_board, and clear the MCAN RX new-message interrupt before draining the FIFO.

This fixes a long-standing issue where heavy CAN traffic during board startup could leave a backlog that delayed all later CAN reception.

本次变更优化了 c_board 与 rmcs_board 的 CAN 接收中断处理，核心目标是消除启动阶段突发流量后残留的 RX 积压与后续接收延迟。

主要改动：

• c_board：Can::handle_uplink 改为在 FIFO0 中仍有待处理报文时持续循环读取并逐条处理，而不是只读一帧；每次处理后清除 FIFO0 读完成标记。
• rmcs_board：Can::handle_uplink 同样改为持续读取 MCAN RX FIFO0，直到队列为空为止。
• rmcs_board：Can::irq_handler 调整了中断标志清除顺序，先处理并清除 RXFIFO0_NEW_MSG，执行接收处理后，再统一清除剩余标志，避免一次性清除带来的处理时序问题。

整体效果是让中断服务期间尽可能“清空”当前积压的 CAN 帧，减少高负载启动场景下的接收滞后。

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 532541f5-0d4c-4e19-9edc-357f7b087695

📥 Commits

Reviewing files that changed from the base of the PR and between ce221e5 and 6718650.

📒 Files selected for processing (2)

• firmware/c_board/app/src/can/can.hpp
• firmware/rmcs_board/app/src/can/can.hpp

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Walkthrough

修改c_board与rmcs_board固件中CAN模块的上行接收处理逻辑，将原先单次读取一条报文的方式改为循环读取直至FIFO为空。同时调整rmcs_board中断处理函数对中断标志的清除时机，改为分阶段清除。未涉及公开接口声明变更。

Changes

CAN上行FIFO批量读取改造

Layer / File(s)	Summary
c_board FIFO0循环读取与解析 firmware/c_board/app/src/can/can.hpp	handle_uplink基于RF0R.FMP0引入while循环，循环体内重新断言寄存器状态、解析RIR/RDTR/RDLR/RDHR并组装数据，调用serializer.write_can后设置RF0R.RFOM0逐条清除FIFO0。
rmcs_board FIFO0循环读取与分阶段标志清除 firmware/rmcs_board/app/src/can/can.hpp	handle_uplink循环调用mcan_read_rxfifo直到返回空状态再逐条写入serializer.write_can；irq_handler引入remaining_flags，先清除并处理RXFIFO0_NEW_MSG标志，再统一清除剩余标志。

Estimated code review effort: 3 (Moderate) | ~20 minutes

Sequence Diagram(s)

sequenceDiagram
  participant IRQ as CAN中断
  participant Handle as handle_uplink
  participant Reg as FIFO寄存器/MCAN接口
  participant Serializer as serializer

  IRQ->>Handle: 触发上行中断处理
  loop 队列非空
    Handle->>Reg: 读取一条报文数据
    Handle->>Serializer: write_can(数据)
    Handle->>Reg: 清除/推进队列指针
  end
  Handle-->>IRQ: 处理完成返回

Loading

Poem

一只兔子蹲在FIFO边，
从前只啃一口就走远，
如今循环嚼到盘底光，
报文颗颗都不留仓，
蹦跳着把标志位轻轻拂 🐰📡

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	标题准确概括了通过清空启动突发后的 CAN 接收积压来消除持续 RX 延迟的核心改动。
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch dev/fix-can

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