Refine Protobuf encoding and decoding

[Kyle-Ye]

Summary

• add fast paths for single-byte varints and short length-delimited fields
• make UInt64 the canonical unsigned decode path and keep signed decoding on the full-width varint path
• reject truncated fixed-width fields instead of silently accepting them
• expand boundary and malformed-input coverage

[codecov]

Codecov Report

❌ Patch coverage is 68.29268% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 14.31%. Comparing base (79f67f5) to head (6c5afca).
⚠️ Report is 39 commits behind head on main.

Files with missing lines	Patch %	Lines
...penSwiftUICore/Data/Protobuf/ProtobufDecoder.swift	73.46%	13 Missing ⚠️
...penSwiftUICore/Data/Protobuf/ProtobufEncoder.swift	58.06%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #990      +/-   ##
==========================================
- Coverage   14.52%   14.31%   -0.21%     
==========================================
  Files         687      691       +4     
  Lines       46292    48892    +2600     
==========================================
+ Hits         6724     6999     +275     
- Misses      39568    41893    +2325     

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