fix: stats semver wire alignment using a zero sentinel

[atharvalade]

Which issue does this PR close?

Closes #2979

Rationale

Stats wire encoding could omit iggy_server_semver when None, while multiple deserializers assume a fixed 4-byte semver slot (0 sentinel), risking downstream field misalignment.

What changed?

When semver was None, the binary stats encoder could skip semver bytes (and in protocol encoder also skip tail fields), which could shift subsequent fields for decoders expecting fixed layout.
Now semver is always encoded as 4 bytes using 0 as the sentinel for None, and tail fields (threads_count, free_disk_space, total_disk_space) are always encoded.
The stats decoder now normalizes semver 0 back to None, and tests were updated/added to lock this layout and sentinel behavior.

Local Execution

• Passed
• Pre-commit hooks not ran

AI Usage

• Opus 4.6
• exploration and identification
• ran cargo fmt --check, cargo clippy, targeted cargo test -p iggy_binary_protocol responses::system::get_stats, and repository CI scripts for trailing whitespace/newline and license headers
• yes all code can be explained

[codecov]

Codecov Report

❌ Patch coverage is 87.09677% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.74%. Comparing base (411a697) to head (f4f152b).

Files with missing lines	Patch %	Lines
.../binary_protocol/src/responses/system/get_stats.rs	87.09%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3036      +/-   ##
============================================
- Coverage     71.74%   71.74%   -0.01%     
  Complexity      943      943              
============================================
  Files          1121     1121              
  Lines         93800    93795       -5     
  Branches      71124    71130       +6     
============================================
- Hits          67301    67291      -10     
+ Misses        23863    23852      -11     
- Partials       2636     2652      +16     

Components	Coverage Δ
Rust Core	72.35% <87.09%> (-0.01%)	⬇️
Java SDK	62.30% <ø> (ø)
C# SDK	67.43% <ø> (-0.21%)	⬇️
Python SDK	81.43% <ø> (ø)
Node SDK	91.44% <ø> (+0.12%)	⬆️
Go SDK	38.68% <ø> (ø)

Files with missing lines	Coverage Δ
.../binary_protocol/src/responses/system/get_stats.rs	86.90% <87.09%> (-0.24%)	⬇️

... and 21 files with indirect coverage changes

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