feat: add fallible MRT message trace iterators

[digizeph]

• Add opt-in record/element trace iterators for outer-to-inner MRT/BGP4MP/BGP inspection.
• Preserve wire metadata needed to diagnose malformed messages: BGP marker, declared length, byte offsets, and framing issues.
• Provide fallible variants that retain decoded outer layers and raw record bytes when semantic parsing fails.
• Keep existing parsed models and iterator behavior unchanged.

Proposed solution

• Add into_trace_record_iter() and into_fallible_trace_record_iter().
• Add element equivalents that attach each BgpElem to its source trace.
• Return structured layers suitable for text output, JSON serialization, and application traversal.

Expected output

MRT [0+76] timestamp=1710000000 type=BGP4MP subtype=MESSAGE_AS4 length=64
  BGP4MP [12+64] peer=192.0.2.1 AS64496 local=192.0.2.2 AS64497 afi=IPv4
    BGP [36+36] marker=ffffffffffffffffffffffffffffffff length=36 type=UPDATE

For malformed messages, the fallible iterator should still expose decoded outer layers and the parse error.