Bidirectional pipe implementation

[GregoryLi360]

Summary

• Lock-free SPSC bidirectional byte pipe over shared memory (no_std compatible)
• RingHeader + RingData in ring.rs — monotonic cursors, Acquire/Release ordering, wrap-around handled internally
• RingProducer / RingConsumer — non-blocking partial write/read with WouldBlock
• BidirectionalPipe — composes two rings with interleaved layout for cache locality, Side::A / Side::B flip writer/reader wiring
• SharedMemoryRegion — single unsafe entry point, safe API downstream
• Interleaved memory layout [HeaderA][DataA][HeaderB][DataB] — headers adjacent to data, separated from each other to avoid false sharing
• ring_size must be a multiple of 8 for header alignment, region must be 8-byte aligned
• Zero-length read/write always returns Ok(0) regardless of ring state
• 27 unit tests covering round-trips, wrap-around, partial read/write, fill/drain/refill, multi-lap, interleaved bidirectional, zero-length edge cases

Test plan

• cargo test passes on fix (27/27)
• Integration with control protocol