ci: add Miri UB detector for binary_protocol and consensus

[krishvishal]

Summary

• Adds a miri task to the pre-merge action, scoped to iggy_binary_protocol and consensus (the unsafe-heavy crates that don't link compio's io_uring driver). Pinned
  nightly-2026-04-21, -Zmiri-tree-borrows -Zmiri-strict-provenance, separate cache key.
• New rust-miri component triggers on core/binary_protocol/** and core/consensus/**. Mirrored in justfile as just miri for local runs.

[codecov]

Codecov Report

❌ Patch coverage is 94.89603% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.13%. Comparing base (99c82b8) to head (8345e34).

Files with missing lines	Patch %	Lines
core/binary_protocol/src/consensus/message.rs	88.15%	0 Missing and 18 partials ⚠️
core/binary_protocol/src/consensus/iobuf.rs	96.95%	7 Missing ⚠️
core/common/src/types/send_messages2.rs	96.42%	0 Missing and 1 partial ⚠️
core/journal/src/prepare_journal.rs	96.55%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3284      +/-   ##
============================================
+ Coverage     73.78%   74.13%   +0.34%     
  Complexity      943      943              
============================================
  Files          1200     1200              
  Lines        109116   109618     +502     
  Branches      86004    86526     +522     
============================================
+ Hits          80516    81268     +752     
+ Misses        25874    25578     -296     
- Partials       2726     2772      +46     

Components	Coverage Δ
Rust Core	75.39% <94.89%> (+0.46%)	⬆️
Java SDK	58.44% <ø> (ø)
C# SDK	69.13% <ø> (-0.33%)	⬇️
Python SDK	81.43% <ø> (ø)
Node SDK	91.41% <ø> (-0.04%)	⬇️
Go SDK	39.91% <ø> (ø)

Files with missing lines	Coverage Δ
core/binary_protocol/src/consensus/command.rs	100.00% <100.00%> (ø)
core/binary_protocol/src/consensus/header.rs	79.68% <100.00%> (+0.87%)	⬆️
core/common/src/alloc/buffer.rs	90.18% <100.00%> (+22.78%)	⬆️
core/common/src/types/send_messages2.rs	47.26% <96.42%> (+2.93%)	⬆️
core/journal/src/prepare_journal.rs	83.49% <96.55%> (+0.71%)	⬆️
core/binary_protocol/src/consensus/iobuf.rs	90.29% <96.95%> (+55.09%)	⬆️
core/binary_protocol/src/consensus/message.rs	82.28% <88.15%> (+33.00%)	⬆️

... and 24 files with indirect coverage changes

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[krishvishal]

/ready