feat(bytesbuf): clamp minimum reservation size in BytesBufWriter

[sandersaares]

Summary

BytesBufWriter (the std::io::Write adapter for BytesBuf) previously reserved only the exact number of bytes requested on each write() call. Since Write is designed for piece-by-piece writing in small increments, this caused excessive memory allocations - each tiny write triggered its own reservation.

Changes

The writer now uses a two-tier minimum reservation strategy via a new ensure_sufficient_capacity() method:

• Empty buffer, small payload (<=4 KiB): reserve 4 KiB. We assume total data will be modest, avoiding waste for small payloads (e.g. short JSON error responses, where over-reserving could open a door to resource exhaustion).
• Otherwise: reserve at least 64 KiB (or the payload size if larger). Once data crosses the 4 KiB boundary we are likely dealing with a large payload, so we take big bites to reduce allocation frequency.

The reservation logic is factored out of write() into ensure_sufficient_capacity() on BytesBufWriter, keeping the Write impl clean and providing a natural home for the design rationale documentation.

Testing

Three new tests covering:

• Empty buffer with small write -> 4 KiB reservation
• Empty buffer with large write (>4 KiB) -> 64 KiB reservation
• Non-empty buffer needing more capacity -> 64 KiB reservation

Closes #271

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (2a15324) to head (bfd668c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #277   +/-   ##
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  Coverage   100.0%   100.0%           
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  Files         141      141           
  Lines        8626     8635    +9     
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+ Hits         8626     8635    +9     

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