CAN: add frame receive timestamp

[xarantolus]

This allows users to implement time synchronization based on CAN timings.

[Copilot]

Pull request overview

Adds reception timestamps to CAN frames to support user-space time synchronization based on CAN SOF timing, by enabling bxCAN timestamp capture and exposing an extended timestamp through the HAL boundary into Rust.

Changes:

• Enable bxCAN time-triggered mode (TTCM) and extend the 16-bit RX SOF timestamp to a per-slot 64-bit counter in the ISR path.
• Extend the C can_frame_t ABI and Rust Frame structs to carry hw_timestamp_rx, and plumb the value through can_receive.
• Add host-side Rust unit tests that specify the timestamp wrap-extension behavior to keep Rust/C logic aligned.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/drivers/can.rs	Adds test-only spec for wrap-extension behavior (and documents timestamp semantics).
machine/cortex-m/st/stm32l4/interface/export.h	Extends can_frame_t with hw_timestamp_rx for RX timestamps.
machine/cortex-m/st/stm32l4/interface/can.c	Enables TTCM and implements 16→64-bit RX timestamp wrap-extension in drain_fifo().
machine/cortex-m/src/stub/can.rs	Mirrors the new timestamp field in the stub Frame.
machine/cortex-m/src/native/can.rs	Mirrors/plumbs the new timestamp field across the Rust↔C boundary.

Comments suppressed due to low confidence (1)

src/drivers/can.rs:259

• This comment treats the wrap period as a fixed ">65.5 ms" gap, but the wrap time depends on CAN bitrate (wrap period = 65536 bit-times, i.e., 65536/bitrate seconds). Consider expressing the limitation in bit-times or in terms of bitrate to keep it correct for non-1Mbps configurations.

    fn idle_gap_longer_than_one_epoch_undercounts_is_known_limitation() {
        // Known caveat: a >65.5 ms RX gap hides full wraps (the `<`
        // rule sees only one). Fine — sync traffic is far faster.
        assert_eq!(run(&[100, 90]), [100, 0x1_0000 + 90]);

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