perf(server-ng): compute batch checksum in a single produce pass

[hubcio]

Produce hashed each batch three times: convert, stamp, and flush
revalidation. The batch checksum is redefined to cover the six header
meta fields plus each message's stored checksum field instead of the
whole blob, binding bodies transitively through the per-message
checksums every validating decode re-verifies. Stamping now hashes N*8
bytes instead of the full blob, and locally originated batches decode
trusted; replicated blobs keep a per-message receive gate on
followers, so transit integrity holds end to end.

Message data written by earlier server-ng builds fails checksum
validation after this change; wipe local_data when upgrading.
Per-message checksums are computed in a single oneshot pass over the
encoded record, pinned by a streaming-vs-oneshot reference test.

This is half of the single-node latency parity work; the consumer-side
sealed-segment read-handle LRU and the numa:auto shard-allocation
default were split out into a separate PR.

[codecov]

Codecov Report

❌ Patch coverage is 91.91176% with 44 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.65%. Comparing base (c51b754) to head (15cebf5).

Files with missing lines	Patch %	Lines
core/server_common/src/send_messages2.rs	94.08%	16 Missing and 15 partials ⚠️
core/partitions/src/iggy_partition.rs	29.41%	10 Missing and 2 partials ⚠️
core/partitions/src/iggy_partitions.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3746       +/-   ##
=============================================
- Coverage     76.20%   63.65%   -12.55%     
  Complexity      969      969               
=============================================
  Files          1325     1324        -1     
  Lines        163090   152239    -10851     
  Branches     135971   125197    -10774     
=============================================
- Hits         124281    96907    -27374     
- Misses        35123    51523    +16400     
- Partials       3686     3809      +123     

Components	Coverage Δ
Rust Core	60.76% <91.91%> (-15.41%)	⬇️
Java SDK	62.71% <ø> (ø)
C# SDK	71.13% <ø> (-1.14%)	⬇️
Python SDK	89.45% <ø> (ø)
PHP SDK	84.52% <ø> (ø)
Node SDK	96.34% <ø> (+0.08%)	⬆️
Go SDK	43.08% <ø> (ø)

Files with missing lines	Coverage Δ
core/partitions/src/journal.rs	77.21% <100.00%> (ø)
core/server-ng/src/responses.rs	39.76% <ø> (ø)
core/simulator/src/client.rs	60.39% <ø> (ø)
core/partitions/src/iggy_partitions.rs	70.02% <0.00%> (ø)
core/partitions/src/iggy_partition.rs	64.09% <29.41%> (-0.19%)	⬇️
core/server_common/src/send_messages2.rs	87.15% <94.08%> (+18.18%)	⬆️

... and 257 files with indirect coverage changes

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[hubcio]

@krishvishal tyvm for review, i think i will split this PR into 2 smaller PRs to make it more bearable for review.

EDIT: done