feat(server): aligned buffer memory pool

[tungtose]

Prepare the memory pool and buffer infrastructure for O_DIRECT I/O. Direct I/O requires buffers to be aligned to the underlying block size (commonly 4096 bytes). This allows the kernel to bypass the page cache, reducing double buffering and giving more predictable I/O latency.

Known Trade-offs:

• Minimum allocation size is now 4096 bytes, meaning small utility buffers (e.g. put_u32_le, put_u64_le) now consume more memory than before
• make_mutable in the HTTP path now copies buffers due to alignment incompatibility with Bytes

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 51 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.26%. Comparing base (3f651e0) to head (a0649cc).

Files with missing lines	Patch %	Lines
core/common/src/alloc/buffer.rs	26.66%	33 Missing ⚠️
core/common/src/alloc/memory_pool.rs	50.00%	13 Missing and 2 partials ⚠️
...ore/common/src/types/message/messages_batch_mut.rs	0.00%	1 Missing ⚠️
...e/common/src/types/segment_storage/index_reader.rs	90.00%	0 Missing and 1 partial ⚠️
...ommon/src/types/segment_storage/messages_reader.rs	90.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2921      +/-   ##
============================================
- Coverage     70.27%   70.26%   -0.01%     
  Complexity      862      862              
============================================
  Files          1028     1028              
  Lines         85279    85302      +23     
  Branches      62656    62689      +33     
============================================
+ Hits          59932    59940       +8     
- Misses        22833    22838       +5     
- Partials       2514     2524      +10     

Flag	Coverage Δ
csharp	67.43% <ø> (-0.19%)	⬇️
go	36.37% <ø> (ø)
java	59.87% <ø> (ø)
node	91.37% <ø> (-0.15%)	⬇️
python	81.43% <ø> (ø)
rust	70.64% <50.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
.../binary/handlers/messages/send_messages_handler.rs	100.00% <ø> (ø)
core/server/src/http/messages.rs	85.00% <100.00%> (+0.38%)	⬆️
...ore/common/src/types/message/messages_batch_mut.rs	48.04% <0.00%> (+0.29%)	⬆️
...e/common/src/types/segment_storage/index_reader.rs	72.81% <90.00%> (-0.43%)	⬇️
...ommon/src/types/segment_storage/messages_reader.rs	81.94% <90.00%> (+4.72%)	⬆️
core/common/src/alloc/memory_pool.rs	69.12% <50.00%> (-1.18%)	⬇️
core/common/src/alloc/buffer.rs	66.29% <26.66%> (-12.49%)	⬇️

... and 11 files with indirect coverage changes

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