feat(partitions): rework partitions using new message

[numinnex]

Refactors partition messaging around Message<H, B>, where the backing storage is part of the type rather than
being encoded through the old TwoHalves prefix/tail layout.

Request messages now use a contiguous Owned backing, while response-style paths can use fragmented storage
directly. This removes the old prefix/tail copy-on-write design entirely, along with the aliasing and uniqueness
problems it introduced. In particular, storage reads no longer need to reconstruct a request-shaped message from
a split buffer, which was fragile once the tail could already be shared by cache-backed Frozen values. The new
design makes the ownership model explicit, keeps the request path simple, and avoids relying on prefix
detachment semantics to preserve correctness.

[codecov]

Codecov Report

❌ Patch coverage is 12.77412% with 1591 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.80%. Comparing base (411a697) to head (cc2ddd6).

Files with missing lines	Patch %	Lines
core/common/src/types/send_messages2.rs	0.00%	408 Missing ⚠️
core/partitions/src/iggy_partitions.rs	0.00%	296 Missing ⚠️
core/iobuf/src/lib.rs	6.21%	181 Missing ⚠️
core/partitions/src/journal.rs	0.00%	169 Missing ⚠️
core/binary_protocol/src/consensus/message.rs	41.88%	110 Missing and 1 partial ⚠️
core/partitions/src/messages_writer.rs	0.00%	78 Missing ⚠️
core/partitions/src/iggy_index_writer.rs	0.00%	56 Missing ⚠️
core/partitions/src/iggy_partition.rs	0.00%	53 Missing ⚠️
core/partitions/src/log.rs	0.00%	30 Missing ⚠️
core/partitions/src/segment.rs	0.00%	29 Missing ⚠️
... and 18 more

❌ Your patch check has failed because the patch coverage (12.77%) is below the target coverage (50.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3020      +/-   ##
============================================
- Coverage     71.74%   70.80%   -0.95%     
  Complexity      943      943              
============================================
  Files          1121     1126       +5     
  Lines         93800    94710     +910     
  Branches      71124    72047     +923     
============================================
- Hits          67301    67063     -238     
- Misses        23863    25002    +1139     
- Partials       2636     2645       +9     

Components	Coverage Δ
Rust Core	71.08% <12.77%> (-1.28%)	⬇️
Java SDK	62.30% <ø> (ø)
C# SDK	67.47% <ø> (-0.17%)	⬇️
Python SDK	81.43% <ø> (ø)
Node SDK	91.53% <ø> (+0.21%)	⬆️
Go SDK	38.68% <ø> (ø)

Files with missing lines	Coverage Δ
core/common/src/error/iggy_error.rs	100.00% <ø> (ø)
core/consensus/src/namespaced_pipeline.rs	97.32% <100.00%> (+0.12%)	⬆️
core/metadata/src/impls/recovery.rs	70.63% <100.00%> (+0.23%)	⬆️
core/shard/src/lib.rs	0.00% <ø> (ø)
core/partitions/src/lib.rs	0.00% <0.00%> (ø)
...ommon/src/types/segment_storage/messages_writer.rs	69.86% <0.00%> (-3.00%)	⬇️
core/metadata/src/stm/mod.rs	45.56% <0.00%> (-1.19%)	⬇️
core/simulator/src/client.rs	0.00% <0.00%> (ø)
core/consensus/src/impls.rs	42.17% <60.00%> (-0.31%)	⬇️
core/consensus/src/plane_mux.rs	0.00% <0.00%> (ø)
... and 22 more

... and 33 files with indirect coverage changes

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[tungtose]

The simulator is currently hanging on the send messages to partition step

[hubcio]

also outside of diff:
offset is Arc (iggy_partition.rs:43) but all stores use Ordering::Relaxed (iggy_partition.rs:108, iggy_partitions.rs:774). the Arc wrapper enables cross-thread sharing - if any other shard/thread reads this (metrics, consumer group coordination), Relaxed provides no ordering guarantees on ARM. should be Release on stores, Acquire on loads at cross-shard boundaries. on x86 this is harmless due to TSO, but iggy targets cross-platform.