Use zerocopy for mrt header parsing

[ties]

Use zerocopy for parsing the MRT header. This potentially saves a number of read syscalls. Personally I also think the pattern is quite readable for parsing structs from the wire.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.09%. Comparing base (5eb9107) to head (96a948f).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #263      +/-   ##
==========================================
- Coverage   91.23%   91.09%   -0.14%     
==========================================
  Files          83       83              
  Lines       14939    14974      +35     
==========================================
+ Hits        13630    13641      +11     
- Misses       1309     1333      +24     

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[Copilot]

Pull request overview

This PR updates MRT common-header parsing/encoding to use zerocopy for struct-based, big-endian field access, aiming to reduce per-field cursor reads and simplify on-wire layout handling.

Changes:

• Introduce RawMrtCommonHeader / RawMrtEtCommonHeader on-wire structs and parse via zerocopy.
• Update CommonHeader::encode() to serialize via the same raw header structs.
• Add Criterion benchmarks for into_raw_record_iter on updates and RIB inputs; add optional zerocopy dependency behind the parser feature.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/parser/mrt/mrt_header.rs	Switch MRT header parsing/encoding to zerocopy-backed raw structs; adjust ET handling to build combined header bytes without BytesMut.
benches/internals.rs	Add benchmark cases for into_raw_record_iter on updates and RIB datasets.
Cargo.toml	Add optional zerocopy dependency and include it in the parser feature.

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[Copilot]

size_of is used in the const assertions but isn’t imported or fully qualified, which will fail to compile. Import core::mem::size_of/std::mem::size_of (or change these calls to core::mem::size_of::<...>()).